Wearable ambulatory medical device with multiple sensing electrodes

ABSTRACT

An ambulatory medical device including a plurality of electrodes configured to be disposed at spaced apart positions about a patient&#39;s body, an electrode signal acquisition circuit, and a monitoring circuit. The acquisition circuit has a plurality of inputs each electrically coupled to a respective electrode of the plurality of electrodes and is configured to sense a respective signal provided by a plurality of different pairings of the plurality of electrodes. The monitoring circuit is electrically coupled to an output of the acquisition circuit and is configured to analyze the respective signal provided by each of the plurality of different pairings and to instruct the acquisition circuit to select at least one of the plurality of different pairings to monitor based on at least one of the quality of the respective signal, a phase difference between the respective signal and that of other pairings, a position of electrodes relative to the patient&#39;s body, and other criteria.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Application Ser. No. 61/345,914 entitled “WEARABLEAMBULATORY MEDICAL DEVICE WITH MULTIPLE SENSING ELECTRODES,” filed May18, 2010, and to U.S. Provisional Application Ser. No. 61/424,344entitled “WEARABLE AMBULATORY MEDICAL DEVICE WITH MULTIPLE SENSINGELECTRODES,” FILED Dec. 17, 2010, each of which is incorporated hereinby reference in its entirety.

BACKGROUND OF INVENTION

1. Field of Invention

The present invention is generally directed to the detection of cardiacfunction in a patient, and more particularly to the detection of cardiacfunction and the treatment of cardiac conditions in an ambulatorymedical device, such as a wearable defibrillator.

2. Discussion of Related Art

With a portable medical device, such as a wearable defibrillator worn byan ambulatory patient, the patient's electrocardiogram (ECG) signal isobtained from body surface electrodes. When the ECG signal is obtainedin this manner, electrical noise or electrode fall-off frequentlydegrades the quality of the ECG signal. The challenge becomes one ofextracting a clean ECG signal from the sometimes noisy signals derivedfrom the body-surface electrodes.

Electrode noise can be caused by electrodes sliding on the patient'sbody due to extreme patient movement, such as vigorous exercise. Noisecan also be caused by a poorly fit electrode belt or garment allowingthe electrodes to slide on the patient's body with minor patientmovement. Electrode fall-off can be caused by the electrodes flippingover and losing contact with the body, or lifting from the body andlosing contact. Even where the electrodes are properly positioned on thepatient's body, excessively dry skin can also cause noise.

Known ambulatory wearable defibrillators, such as the LifeVest® WearableCardioverter Defibrillator available from Zoll Medical Corporation ofChelmsford, Mass., use four ECG sensing electrodes in a dual-channelconfiguration. That is, an electrical signal provided by one of the fourECG sensing electrodes is paired with the electrical signal provided byanother of the four ECG sensing electrodes to form a channel. Thisarrangement of ECG sensing electrodes is usually suitable because inmost cases it is rare that noise or electrode movement affects theentire body circumference. The dual-channel configuration providesredundancy and allows the system to operate on a single channel ifnecessary, when one of the channels is declared unusable due to ECGsensing electrode fall-off, or to an inferior signal-to-noise ratio.Because signal quality also varies from patient to patient, having twochannels provides the opportunity to have improved signal pickup, sincethe ECG sensing electrodes are located in different body positions. Thetwo channel system also allows analysis of the ECG signal to determinecardiac conditions as described in U.S. Pat. No. 5,944,669.

A problem with existing electrode systems used in ambulatory medicaltreatment devices, such as a wearable defibrillator, is that there arestill instances where there is noise on both channels. When there isnoise or fall-off, the device issues alarms so that the patient can takeaction to correct the problem. With a noisy ECG signal, the arrhythmiadetection algorithm in the wearable defibrillator device can be “fooled”into detecting the noise as an arrhythmia, thereby causing the device toissue a treatment sequence that, if not terminated by the patient, coulddeliver an unnecessary shock.

SUMMARY OF INVENTION

Embodiments of the present invention are directed to a wearable medicalmonitoring device and/or to a wearable medical monitoring and treatmentdevice that incorporates multiple ECG sensing electrodes disposed atdifferent axial positions around the body of a patient and that canchoose from multiple channels corresponding to different pairings ofthose multiple ECG sensing electrodes to vastly improve the quality ofthe ECG signal obtained. This improved ECG sensor design can be used toreduce noise, to reduce the number of fall-off alarms, to reduce thenumber of cardiac arrhythmia false detections, or all of the above. Themultiple channels provide different views of the heart's electricalactivity and can be used to improve the detection sensitivity andspecificity.

In accordance with one aspect of the present invention, an ambulatorymedical device is provided. In one embodiment, the ambulatory medicaldevice comprises a plurality of electrodes configured to be disposed atspaced apart positions about a body of a patient, an electrode signalacquisition circuit, and a monitoring circuit. The electrode signalacquisition circuit has a plurality of inputs, each respective input ofthe plurality of inputs being electrically coupled to a respectiveelectrode of the plurality of electrodes. The electrode signalacquisition circuit is configured to sense a respective signal providedby a plurality of different pairings of the plurality of electrodes. Themonitoring circuit is electrically coupled to an output of the electrodesignal acquisition circuit. The monitoring circuit is configured toanalyze the respective signal provided by each of the plurality ofdifferent pairings and to instruct the electrode signal acquisitioncircuit to select at least one of the plurality of different pairings tomonitor based upon at least one of a quality of the respective signalprovided by the selected at least one of the plurality of differentpairings, a phase difference between the respective signal provided bythe selected at least one of the plurality of different pairings and therespective signal provided by another selected at least one of theplurality of different pairings, a position of the respective electrodesof the selected at least one of the plurality of different pairingsrelative to the body of the patient, a plane defined by the respectiveelectrodes of the selected at least one of the plurality of differentpairings, and a cardiac cycle of a heart of the patient.

In accordance with one embodiment, the ambulatory medical device furthercomprises a garment that is configured to be worn about the body of thepatient, and the plurality of electrodes are integrated into thegarment. In accordance with another embodiment, the plurality ofelectrodes are ECG sensing electrodes, and the monitoring circuit is acardiac monitoring and arrhythmia detection circuit.

In one embodiment, the plurality of ECG sensing electrodes includes atleast three ECG sensing electrodes. In another embodiment, the pluralityof ECG sensing electrodes are not all located in a common plane.

In one embodiment, the cardiac monitoring and arrhythmia detectioncircuit is configured to analyze the respective signal provided by eachof the plurality of different pairings and to instruct the electrodesignal acquisition circuit to select the at least one of the pluralityof different pairings to monitor based upon the quality of therespective signal provided by the selected at least one of the pluralityof different pairings and the phase difference between the respectivesignal provided by the selected at least one of the plurality ofdifferent pairings and the respective signal provided by the otherselected at least one of the plurality of different pairings. Inaccordance with a further aspect of this embodiment, the ambulatorymedical device further comprises a plurality of therapy electrodesintegrated into the garment and configured to deliver a defibrillatingshock to the body of the patient in response to detection of a treatablecardiac arrhythmia by the cardiac monitoring and arrhythmia detectioncircuit.

In accordance with another embodiment, the cardiac monitoring andarrhythmia detection circuit is configured to analyze the respectivesignal provided by each of the plurality of different pairings and toinstruct the electrode signal acquisition circuit to select the at leastone of the plurality of different pairings to monitor based upon thequality of the respective signal provided by the selected at least oneof the plurality of different pairings and the plane defined by therespective electrodes of the selected at least one of the plurality ofdifferent pairings.

In another embodiment, the cardiac monitoring and arrhythmia detectioncircuit is configured to analyze the respective signal provided by eachof the plurality of different pairings and to instruct the electrodesignal acquisition circuit to select the at least one of the pluralityof different pairings to monitor based upon the position of therespective electrodes of the selected at least one of the plurality ofdifferent pairings relative to the body of the patient and the cardiaccycle of the heart of the patient.

In an alternative embodiment, the cardiac monitoring and arrhythmiadetection circuit is configured to select at least three of theplurality of different pairings to monitor based upon the at least oneof the quality of the respective signal provided by each of the selectedat least three of the plurality of different pairings, the phasedifference between the respective signal provided by each of theselected at least three of the plurality of different pairings, theposition of the respective electrodes of the selected at three of theplurality of different pairings relative to the body of the patient, theplane defined by the respective electrodes of the selected at leastthree of the plurality of different pairings, and the cardiac cycle ofthe heart of the patient.

In one embodiment, the plurality of electrodes includes at least fourECG sensing electrode that are not all located in a common plane.

In accordance with one embodiment, the monitoring circuit is configuredto analyze the respective signal provided by each of the plurality ofdifferent pairings and to instruct the electrode signal acquisitioncircuit to select the at least one of the plurality different pairingsto monitor based upon the quality of the respective signal provided bythe selected at least one of the plurality of different pairings and thephase difference between the respective signal provided by the selectedat least one of the plurality of different pairings and the respectivesignal provided by the other selected at least one of the plurality ofdifferent pairings.

In accordance with another embodiment, the monitoring circuit isconfigured to analyze the respective signal provided by each of theplurality of different pairings and to instruct the electrode signalacquisition circuit to select the at least one of the plurality ofdifferent pairings to monitor based upon the quality of the respectivesignal provided by the selected at least one of the plurality ofdifferent pairings and the plane defined by the respective electrodes ofthe selected at least one of the plurality of different pairings.

In accordance with yet another embodiment, the monitoring circuit isconfigured to analyze the respective signal provided by each of theplurality of different pairings and to instruct the electrode signalacquisition circuit to select the at least one of the plurality ofdifferent pairings to monitor based upon the position of the respectiveelectrodes of the selected at least one of the plurality of differentpairings relative to the body of the patient and the cardiac cycle ofthe heart of the patient.

In accordance with one embodiment, the monitoring circuit is configuredto select at least three of the plurality of different pairings tomonitor based upon the at least one of the quality of the respectivesignal provided by each of the selected at least three of the pluralityof different pairings, the phase difference between the respectivesignal provided by each of the selected at least three of the pluralityof different pairings, the position of the respective electrodes of theselected at three of the plurality of different pairings relative to thebody of the patient, the plane defined by the respective electrodes ofthe selected at least three of the plurality of different pairings, andthe cardiac cycle of the heart of the patient. In accordance with afurther embodiment, the monitoring circuit is configured to select atleast two of the selected at least three of the plurality of differentpairings to monitor during a first time interval and to select adifferent at least two of the selected at least three of the pluralityof different pairings to monitor during a second time interval.

In one embodiment, the plurality of ECG sensing electrodes includes atleast three ECG sensing electrodes.

In accordance with one embodiment, the electrode signal acquisitioncircuit includes a selection circuit and a plurality of differentialamplifiers. The selection circuit has a plurality of inputs and aplurality of outputs, each respective input of the plurality of inputsof the selection circuit being electrically coupled to a respective oneof the plurality of electrodes. Each respective differential amplifierof the plurality of differential amplifiers has a pair of inputs and anoutput, each respective input of the pair of inputs being electricallycoupled to a respective one of the plurality of outputs of the selectioncircuit, each respective output of the plurality of differentialamplifiers providing an output signal corresponding to a differencebetween the pair of inputs of the respective differential amplifier.

In accordance with another embodiment in which the electrode signalacquisition circuit includes a plurality of differential amplifiers anda selection circuit, the plurality of differential amplifiers includes arespective differential amplifier for each unique pairing of theplurality of electrodes. In this embodiment, the selection circuit isconfigured to select at least one output of the plurality ofdifferential amplifiers to provide to the monitoring circuit.

In accordance with another embodiment, the electrode signal acquisitioncircuit includes an analog multiplexer and an analog-to-digitalconverter. The analog multiplexor has a plurality of inputs and anoutput, each of the plurality of inputs being electrically coupled to arespective one of the plurality of electrodes, and the analog-to-digitalconverter has an input electrically coupled to the output of the analogmultiplexer. In accordance with an aspect of this embodiment, theanalog-to digital converter has a sampling rate that is at least N timesa desired sampling rate of a signal provided by each of the plurality ofelectrodes, where N is the number of the plurality of electrodes thatare to be monitored. In accordance with another aspect of thisembodiment, the monitoring circuit includes at least one processorconfigured to receive a first digital signal corresponding to a firstelectrode of the plurality of electrodes and a second digital signalcorresponding to a second electrode of the plurality of electrodes, toinvert one of the first and second digital signals and sum the invertedone of the first and second digital signals with the other of the firstand second digital signals to analyze the respective signal provided bya pairing of the first electrode and the second electrode.

In accordance with another embodiment, the electrode signal acquisitioncircuit includes a plurality of analog-to-digital converters, eachrespective analog-to-digital converter having a respective input that iselectrically coupled to a respective one of the plurality of electrodes.In one embodiment, each of the plurality of analog-to-digital convertersis connected to another of the plurality of analog-to-digital convertersby a serial bus.

In accordance with another aspect of the present invention, a method ofmonitoring ECG signals is provided. In one embodiment, the methodcomprises selecting, from among a plurality of ECG sensors, a pluralityof different pairings of ECG sensors; analyzing a respective ECG signalprovided by each of the plurality of different pairings; identifying atleast one of the plurality of different pairings to monitor based uponat least one of a quality of the respective ECG signal provided by theidentified at least one of the plurality of different pairings, a phasedifference between the respective ECG signal provided by the identifiedat least one of the plurality of different pairings and the respectiveECG signal provided by another identified at least one of the pluralityof different pairings, a position of respective ECG sensors of theidentified at least one of the plurality of different pairings relativeto a body of a patient, a plane defined by the respective ECG sensors ofthe identified at least one of the plurality of different pairings, anda cardiac cycle of a heart of the patient; and monitoring the identifiedat least one of the plurality of different pairings.

In accordance with one embodiment, the act of identifying at least oneof the plurality of different pairings to monitor is based upon thequality of the respective ECG signal provided by the identified at leastone of the plurality of different pairings and the phase differencebetween the respective ECG signal provided by the identified at leastone of the plurality of different pairings and the respective ECG signalprovided by the other identified at least one of the plurality ofdifferent pairings.

In one embodiment, the act of selecting the plurality of differentpairings of ECG sensors from among the plurality of ECG sensors includesan act of selecting, from among the plurality of ECG sensors, eachunique pairing of ECG sensors, and the act of analyzing the respectiveECG signal provided by each of the plurality of different pairingsincludes analyzing the respective ECG signal provided by each uniquepairing of ECG sensors.

In a further embodiment, the act of monitoring includes monitoring theidentified at least one of the plurality of different pairings to detecta cardiac arrhythmia. In accordance with one embodiment, the methodfurther comprises acts of detecting the cardiac arrhythmia responsive tothe act of monitoring; determining that the detected cardiac arrhythmiais a type of cardiac arrhythmia that can be treated by applyingdefibrillation to the body of the patient; and applying at least onedefibrillation pulse to the body of the patient.

In another embodiment, the method further comprises acts of detectingthe cardiac arrhythmia responsive to the act of monitoring; selecting atleast one additional pairing of ECG sensors in response to detecting thecardiac arrhythmia and analyzing the respective ECG signal provided bythe at least one additional pairing; determining that the detectedcardiac arrhythmia is also present on the respective ECG signal of theat least one additional pairing; determining that the detected cardiacarrhythmia is a type of cardiac arrhythmia that can be treated byapplying defibrillation to the body of the patient; and applying atleast one defibrillation pulse to the body of the patient.

In an alternative embodiment, the method further comprises acts ofdetecting the cardiac arrhythmia responsive to the act of monitoring;selecting at least one additional pairing of ECG sensors in response todetecting the cardiac arrhythmia and analyzing the respective ECG signalprovided by the at least one additional pairing; determining that thedetected cardiac arrhythmia is also present on the respective ECG signalof the at least one additional pairing; and increasing a confidencelevel that the cardiac arrhythmia has been detected.

In another embodiment, the method further comprises acts of detectingthe cardiac arrhythmia responsive to the act of monitoring; selecting atleast one additional pairing of ECG sensors in response to detecting thecardiac arrhythmia and analyzing the respective ECG signal provided bythe at least one additional pairing; determining that the detectedcardiac arrhythmia is not present on the respective ECG signal of the atleast one additional pairing; and decreasing a confidence level that thecardiac arrhythmia has been detected.

In accordance with one embodiment, the acts of selecting, analyzing, andidentifying are repeated at periodic intervals.

In accordance with another embodiment, the plurality of ECG sensors areintegrated in a garment that is worn about the body of the patient, andthe acts of selecting, analyzing, and identifying are performed eachtime the garment is placed about the body of the patient.

In accordance with another embodiment in which the plurality of ECGsensors are integrated in a garment that is worn about the body of thepatient, the method further comprises an act of detecting strenuousphysical activity of the patient, and repeating the acts of selecting,analyzing, and identifying in response to the act of detecting thestrenuous activity of the patient.

In accordance with another embodiment, the method further comprises actsof determining that the quality of the respective ECG signal provided bya first pairing of ECG sensors of the identified at least one of theplurality of different pairings is below a determined threshold;selecting another paring of ECG sensors to replace the first pairing ofECG sensors; and monitoring the other pairing of ECG sensors.

In accordance with yet another embodiment, the method further comprisesacts of determining, from the quality of the respective ECG signalprovided by a first pairing of ECG sensors of the identified at leastone of the plurality of different pairings, that one or more of the ECGsensors of the first pairing may have at least partially lost contactwith the body of the patient; selecting another paring of ECG sensors toreplace the first pairing of ECG sensors; and monitoring the otherpairing of ECG sensors.

In accordance with one embodiment, the act of identifying at least oneof the plurality of different pairings to monitor is based upon thequality of the respective ECG signal provided by the identified at leastone of the plurality of different pairings and the plane defined by therespective ECG sensors of the identified at least one of the pluralityof different pairings.

In accordance with another embodiment, the act of identifying at leastone of the plurality of different pairings to monitor is based upon theposition of respective ECG sensors of the identified at least one of theplurality of different pairings relative to the body of the patient andthe cardiac cycle of the heart of the patient.

Still other aspects, embodiments, and advantages of these exemplaryaspects and embodiments, are discussed in detail below. Moreover, it isto be understood that both the foregoing information and the followingdetailed description are merely illustrative examples of various aspectsand embodiments of the present invention, and are intended to provide anoverview or framework for understanding the nature and character of theclaimed aspects and embodiments. Any embodiment disclosed herein may becombined with any other embodiment in any manner consistent with atleast one aspect of the invention disclosed herein, and references to“an embodiment,” “some embodiments,” “an alternate embodiment,” “variousembodiments,” “one embodiment,” “at least one embodiment,” “this andother embodiments” or the like are not necessarily mutually exclusiveand are intended to indicate that a particular feature, structure, orcharacteristic described in connection with the embodiment may beincluded in at least one embodiment. The appearances of such termsherein are not necessarily all referring to the same embodiment.Furthermore, in the event of inconsistent usages of terms between thisdocument and documents incorporate herein by reference, the term usagein the incorporated references is supplementary to that of thisdocument; for irreconcilable inconsistencies, the term usage in thisdocument controls.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings are not intended to be drawn to scale. In thedrawings, each identical or nearly identical component that isillustrated in various figures is represented by a like numeral. Forpurposes of clarity, not every component may be labeled in everydrawing. In the drawings:

FIG. 1A illustrates an electrode system of a portable medical device inwhich a plurality of ECG sensing electrodes are integrated into agarment, such as a shirt or vest that can be worn on the body of thepatient, and in which the electrodes are generally disposed in a planeof the patient's heart;

FIG. 1B illustrates an alternative electrode system of a portablemedical device in which the plurality of ECG sensing electrodes areintegrated into a garment such as that depicted in FIG. 1A, but in whichthe electrodes are distributed about the torso of the patient;

FIG. 1C illustrates an alternative electrode system of a portablemedical device in which the plurality of ECG sensing electrodes areintegrated into a garment, such as a belt that can be worn on the bodyof the patient, and in which the electrodes are generally disposed in aplane of the patient's heart;

FIG. 1D illustrates an alternative electrode system of a portablemedical device in which the plurality of ECG sensing electrodes areintegrated into a garment, such as a belt that can be worn on the bodyof the patient, and in which the electrodes are distributed about aportion of the torso of the patient;

FIG. 1E illustrates a further alternative electrode system of a portablemedical device in which the plurality of ECG sensing electrodes areintegrated into a garment, such as a harness, and in which theelectrodes are disposed about the torso of the patient;

FIG. 1F illustrates an alternative electrode system of a portablemedical device in which the plurality of ECG sensing electrodes aredirectly attached to the patient's torso;

FIG. 1G illustrates a plan view of the electrode systems of FIGS. 1A-F;

FIG. 2A illustrates an electrode signal acquisition circuit that may beused with embodiments of the present invention to select, from among aplurality of different pairings of ECG sensing electrodes, thosepairings of electrodes that maximize the signal-to-noise ratio andmaximize the phase discrimination provided by the electrodes;

FIG. 2B illustrates an electrode signal acquisition circuit according toanother embodiment of the present invention that is similar to that ofFIG. 2A but includes the ability to permit one or more the ECG sensingelectrodes to be used as a driven ground electrode;

FIG. 2C illustrates an electrode signal acquisition circuit according toanother embodiment of the present invention that is similar to that ofFIG. 2B and permits one or more the ECG sensing electrodes to be used asa driven ground electrode;

FIG. 3 illustrates an alternative electrode signal acquisition circuitthat may be used with embodiments of the present invention;

FIG. 4 is a functional block diagram of a control unit that may be usedwith embodiments of present invention;

FIG. 5 is a flow diagram of an electrode selection process that may beperformed by at least one processor of the control unit of FIG. 4;

FIG. 6 is a flow diagram of a noise/fall-off detection process that maybe executed by the at least one processor of the control unit of FIG. 4;

FIG. 7 is a flow diagram of a monitoring and analysis routine that maybe executed by the at least one processor of the control unit of FIG. 4;

FIG. 8 is a flow diagram of an alternative monitoring and analysisroutine that may be executed by the at least one processor of thecontrol unit of FIG. 4;

FIG. 9 illustrates a further alternative signal acquisition circuit thatmay be used with embodiments of the present invention;

FIG. 10 illustrates yet another alternative signal acquisition circuitthat may be used with embodiments of the present invention; and

FIG. 11 illustrates another alternative signal acquisition circuit thatmay be used with embodiments of the present invention.

DETAILED DESCRIPTION

This invention is not limited in its application to the details ofconstruction and the arrangement of components set forth in thefollowing description or illustrated in the drawings. The invention iscapable of other embodiments and of being practiced or of being carriedout in various ways. Also, the phraseology and terminology used hereinis for the purpose of description and should not be regarded aslimiting. The use of “including,” “comprising,” “having,” “containing,”“involving,” and variations thereof herein is meant to encompass theitems listed thereafter and equivalents thereof as well as additionalitems.

U.S. Pat. No. 5,944,669, which is incorporated herein by reference inits entirety, describes a method and apparatus for sensing cardiacfunction in a patient that may be used to initiate treatment of adetected cardiac condition. ECG sensing electrodes are used to obtainECG signals from the heart of the patient and those ECG signals areanalyzed using various techniques to provide information indicative ofthe operation of the patient's heart, and whether a treatable cardiaccondition is present for which treatment, such as defibrillation, shouldbe initiated. As described therein, a plurality of pairs of ECG sensingelectrodes are used, such that signals received from the different pairsof ECG sensing electrodes may be compared to one another to improvereliability or detection, so that noise present on one or more of theelectrodes can be identified, so that monitoring may be provided even inthe event that one or more of the sensing electrodes falls off, etc.

Embodiments of the present invention are directed to an electrode systemthat may be used in a wearable medical device, such as that described inU.S. Pat. No. 5,944,669 (hereinafter “the '669 patent”), to monitorcardiac function, to initiate treatment of a detected cardiac condition,or both. As described in more detail below, although embodiments of thepresent invention are primarily described in terms of monitoring signalsfrom a plurality of ECG sensing electrodes, it should be appreciatedthat the techniques described herein may readily be extended for usewith other types of sensors, other than ECG sensing electrodes. Forexample, other types of sensors may include activity sensors, such asmultiple axis accelerometers, pulse oxygen sensors, temperature sensors,respiratory rate sensors, thoracic impedance sensors, blood pressuresensors, acoustic sensors, etc.

As shown in FIG. 1A, in one embodiment of the present invention, theelectrode system 100 includes a plurality of ECG sensing electrodes 10that are disposed at different axial positions around the body of apatient and integrated into a garment 20 a, such as a shirt or vestwhich is worn on the torso of the patient. As depicted in FIG. 1A (aswell as in FIGS. 1B-1F), those ECG electrodes shown in solid line formare disposed on the front of the patient's body, while those ECGelectrodes shown in dotted line form are disposed on the back of thepatient's body. It should be appreciated that although not depicted inthe figures, the plurality of ECG sensing electrodes 10 will generallyinclude electrodes disposed on the sides of the patient's body, as wellas electrodes disposed on the front and back of the patient's body.

The plurality of ECG sensing electrodes 10 may be discrete, dry-sensingcapacitive or conductive electrodes that are, for example, attached tothe garment 20 a by an adhesive or hook and loop fastener, magneticallyattached to the garment 20 a, or alternatively, sewn into the garment 20a. Alternatively still, some or all of the ECG sensing electrodes may beformed from electrically conductive threads sewn into the garment 20 a,such as described in U.S. patent application Ser. No. 13/109,079,entitled “WEARABLE THERAPEUTIC DEVICE,” filed on May 17, 2011, which isincorporated herein by reference in its entirety. It should beappreciated that the present invention is not limited to a particulartype of ECG sensing electrode or method of attachment, as various typesof ECG sensing electrodes, including wet ECG sensing electrodes, andvarious methods of attachment, including adhesive attachment to thepatient's body may be used. Moreover, although embodiments of thepresent invention are primarily described with respect to ECG sensingelectrodes that are electrically coupled to a control unit by wires, thepresent invention is not so limited, as embodiments of the presentinvention may also be used with ECG sensing electrodes that communicatewith a control unit using a wireless communication interface andprotocol, such as Bluetooth, Wireless USB, ZigBee, Wireless Ethernet,GSM, etc. as discussed further below.

As shown in FIG. 1A, the plurality of ECG sensing electrodes 10 aredeployed about the body of the patient at spaced apart axial positionsgenerally located in a plane of the heart of the patient. In accordancewith one embodiment of the present invention, the ECG sensing electrodes10 are deployed about the body of the patient in pairs of generallyopposed electrodes (e.g., ECG sensing electrodes 10 a, 10 b) that areintegrated into a garment 20 a, such as a shirt or vest, although thepresent invention is not so limited. It should be appreciated that thelocation of the plurality of ECG sensing electrodes 10 may be varied toavoid placing an electrode in a location where it could promotediscomfort for the patient, such as directly on the spine of thepatient. Insulated lead wires 15 electrically couple each ECG sensingelectrode of the plurality of ECG sensing electrodes 10 to a controlunit 30 that may include a signal acquisition circuit, such as thatdescribed in more detail with respect to FIGS. 2A-C, 3, 9, 10, and 11below. Although not shown, each of the insulated lead wires 15 may beelectrically connected to a connector that is received in a receptacleof the control unit 30. The control unit 30 may be attached to thegarment 20 a, attached to a belt, received in a holster, or attached toa clip so that it may be easily worn by the patient, or the control unit30 may be carried with the patient in any other convenient manner. Asshown, the electrode system 100 also includes at least one driven groundelectrode 12 that is attached to the garment 20 a and is electricallycoupled to the control unit 30 by an insulated lead wire 18. The atleast one driven ground electrode 12 may be used in the manner describedin the '669 patent to reduce the effects of noise and/or detect if anECG sensing electrode has fallen off. Although the use of a drivenground electrode is preferred to cancel the effects of noise, it shouldbe appreciated that the electrode 12 need not be actively driven, andcould simply be a passive circuit ground.

FIG. 1B illustrates an electrode system 100 in accordance with analternative embodiment of the present invention in which the pluralityof ECG sensing electrodes 10 are again integrated into a garment 20 a,such as a shirt or vest that is worn over the torso of a patient.However, in the embodiment depicted in FIG. 1B, the plurality of ECGsensing electrodes 10 are distributed about the torso of the patient,rather than being generally located in a plane of the patient's heart.As in the embodiment of FIG. 1A, the plurality of ECG sensing electrodes10 are electrically coupled to a control unit 30 by a respectiveelectrically insulated lead wire 15 (not all of which are shown for easeof illustration), although wireless ECG sensing electrodes couldalternatively be used. The electrode system 100 also includes at leastone driven ground electrode 12 that is attached to the garment 20 a andelectrically coupled to the control unit by an insulated lead wire 18.Although only one driven ground electrode 12 is illustrated in thefigures, it should be appreciated that multiple driven ground electrodesmay be used. For example, multiple driven ground electrodes may beprovided, with one of the driven ground electrodes being used withcertain pairings of ECG sensing electrodes, and another of the drivenground electrodes being used with other pairings of ECG sensingelectrodes. Moreover in certain embodiments, the at least one drivenground electrode may be switched to be used with different pairings ofECG sensing electrodes. For example, if it were determined that one ofthe driven ground electrodes had fallen off or had poor contact with thebody of the patient, another of the driven ground electrodes could beused instead.

The plurality of ECG sensing electrodes 10 may be deployed in pairs(e.g., ECG electrodes 10 a, 10 b) of generally opposed electrodes, orsimply spaced apart about the torso of the patient. Although not shownin FIG. 1B, the plurality of ECG sensing electrodes 10 may include afirst grouping of electrodes that are located at spaced apart axialpositions generally located in the plane of the patient's heart, and asecond grouping of electrodes that are located at varying position aboutthe torso of the patient. As discussed in more detail further below, thepresence of ECG sensing electrodes that are not all co-located in theplane of the patient's heart permits the selection of different pairingsof ECG sensing electrodes that can correspond to different planesintersecting the patient's heart. Moreover, although not depicted inFIG. 1B, the plurality of ECG sensing electrodes 10 may include ECGsensing electrodes positioned on side of the patient's torso, as well asthe patient's front and back. As the embodiment of FIG. 1B issubstantially similar to that of FIG. 1A, further discussion of thoseelements common to both embodiments is omitted herein.

FIG. 1C illustrates an electrode system 100 in accordance with a furtherembodiment of the present invention in which the plurality of ECGsensing electrodes 10 are again disposed at different axial positionsaround the body of a patient and generally located in a plane of theheart of the patient as in FIG. 1A. However, in the embodiment depictedin FIG. 1C, the plurality of ECG sensing electrodes 10 are integratedinto a garment 20 b, such as a belt, that is worn about the torso of thepatient. As with the electrode system of FIGS. 1A and 1B, the electrodesystem 100 of FIG. 1C may include any type of ECG sensing electrodes,such as discrete, dry-sensing capacitive or conductive electrodes thatare attached to the garment 20 b by an adhesive or fastener,magnetically attached to the garment 20 b, or sewn into the garment 20b. As with the embodiment of FIGS. 1A and 1B, the plurality of ECGsensing electrodes 10 may be deployed about the patient's body in pairsof generally opposed electrodes (e.g., ECG sensing electrodes 10 a, 10b), or may simply be axially spaced about the torso of the patient. Leadwires 15 (not all of which are shown for ease of illustration)electrically couple each ECG sensing electrode of the plurality of ECGsensing electrodes 10 to the control unit 30, which may be worn on orcarried with the body of the patient. It should be appreciated thatwireless ECG sensing electrodes could alternatively be used. As in theembodiments described previously with respect to FIGS. 1A and 1B, theelectrode system 100 of FIG. 1C also includes at least one driven groundelectrode 12 that is attached to the garment 20 b and may be used toreduce the effects of noise and/or detect if an ECG sensing electrodehas fallen off. The at least one driven ground electrode 12 may bedisposed in the same plane as the plurality of ECG sensing electrodes10, or may be located in a different plane. In some embodiments, the atleast one driven ground electrode 12 may also be used as a therapyelectrode to administer an electrical shock to the heart of the patient,as discussed further with respect to FIG. 1E below. Although notdepicted in FIG. 1C, the plurality of ECG sensing electrodes 10 mayinclude ECG sensing electrodes positioned on side of the patient'storso, as well as the patient's front and back.

FIG. 1D illustrates an electrode system 100 in accordance with yetanother embodiment of the present invention in which the plurality ofECG sensing electrodes 10 are again integrated into a garment 20 b, suchas a belt that is worn over the torso of a patient, but in which theplurality of ECG sensing electrodes 10 are distributed more widely aboutthe torso of the patient. As in the embodiment of FIGS. 1A-C, theplurality of ECG sensing electrodes 10 are electrically coupled to acontrol unit 30 by a respective electrically insulated lead wire 15 (notall of which are shown for ease of illustration), and the electrodesystem 100 also includes at least one driven ground electrode 12. The atleast one driven ground electrode 12 may include a first driven groundelectrode that is generally disposed in a same plane as a plurality ofthe ECG sensing electrodes, and a second driven ground electrodedisposed in a different plane. The plurality of ECG sensing electrodes10 may be deployed in pairs (e.g., ECG electrodes 10 a, 10 b), or simplyspaced apart about the torso of the patient. As the embodiment of FIG.1D is substantially similar to that of FIGS. 1A-C, further discussion ofthose elements that are in common with FIGS. 1A-C is omitted herein.

FIG. 1E illustrates an electrode system 100 in accordance with analternative embodiment of the present invention in which the pluralityof ECG sensing electrodes 10 are again integrated into a garment 20 c.However, in this embodiment, the garment 20 c is constructed in the formof a harness, such as that used in the LifeVest® Wearable CardioverterDefibrillator. The harness includes an adjustable belt 110 andadjustable shoulder straps 120 that permit the harness be easilyadjusted to fit different body types and sizes.

As shown in FIG. 1E, the plurality of ECG sensing electrodes 10 aredisposed about the body of the patient at varying locations along thebelt 110 and shoulder straps 120 of the garment 20 c. As with thepreviously described embodiments, the plurality of ECG sensingelectrodes 10 may be discrete, dry-sensing capacitive or conductiveelectrodes that are attached to the garment 20 c, for example, by anadhesive, by hook and loop fasteners, or by sewing, or alternatively,the electrodes may be formed from electrically conductive threads sewninto the garment 20 c. As with the previously described embodiments, theplurality of ECG sensing electrodes 10 may be deployed in pairs ofgenerally opposed electrodes (e.g., ECG sensing electrodes 10 a and 10b), or alternatively, may not be deployed in pairs, but simply disposedat various locations about the body of the patient. Although notdepicted in FIG. 1E, the plurality of ECG sensing electrodes 10 willgenerally include ECG sensing electrodes disposed on the sides of thepatient's body, as well as on the front and back of the patient's body.

As with the embodiments of FIGS. 1A-D, the electrode system 100 of FIG.1E may include a control unit 30 that is electrically coupled to each ofthe plurality of ECG sensing electrodes 10 by a respective insulatedlead wire 15, although wireless ECG sensing electrodes couldalternatively be used. The control unit 30 may include a signalacquisition circuit, such as that described in more detail with respectto FIGS. 2A-C, 3, 9, 10, and 11, and the control unit may also include acontroller that may not only monitor the ECG signals from the patient,but may analyze those ECG signals and initiate electrical shock therapyto the patient in the event that such treatment is warranted. Thecontrol unit 30 may be integrated into the garment 20 c, attached to thebelt 110, received in a holster (not shown), or attached to a clip sothat it may be easily worn by the patient, or the control unit 30 may becarried with the patient in any other convenient manner.

As with the previously described embodiments, the electrode system 100also includes at least one driven ground electrode 12. As illustrated inFIG. 1E, in at least one embodiment, the at least one driven groundelectrode includes three driven ground electrodes 12 a, 12 b, and 12 c.The driven ground electrodes 12 a-c may be used in the manner describedin the '669 patent to reduce the effects of noise and/or detect if anECG sensing electrode has fallen off. In one embodiment of the presentinvention, the driven ground electrodes 12 a-c can also be used as atherapy electrodes to deliver a defibrillating shock to the body of thepatient, where such treatment is warranted. In this embodiment, theelectrodes 12 a and 12 b are electrically coupled together and act as afirst therapy electrode, with electrode 12 c acting as a second therapyelectrode. The use of two therapy electrodes permits a biphasic shock tobe delivered to the body of the patient, such that a first of the twotherapy electrodes delivers a first phase of the biphasic shock with theother therapy electrode acting as a return, and the other therapyelectrode delivers the second phase of the biphasic shock with the firsttherapy electrode acting as the return. It should be appreciated that insome embodiments, a monophasic shock or other type of defibrillatingpulse of energy may be used.

FIG. 1F illustrates an electrode system 100 in accordance with anotherembodiment of the present invention that includes a plurality of ECGsensing electrodes 10. In contrast to the embodiments described withrespect to FIGS. 1A-E, rather than being attached or integrated into agarment 20 a, 20 b, or 20 c, each of the plurality of ECG sensingelectrodes 10 and the at least one driven ground electrode 12 isdirectly attached to the body of the patient. As with the previouslydescribed embodiments, each of the plurality of ECG sensing electrodes10 is electrically coupled to the control unit 30 by a respectiveinsulating wire 15 (not all of which are shown for ease ofillustration), and the electrode system 100 includes at least one drivenground electrode 12 that is electrically coupled to the control unit 30by an insulated lead wire 18. In accordance with an aspect of thepresent invention, the control unit 30 may include a signal acquisitioncircuit that is capable of selecting, from among the plurality of ECGsensing electrodes 10, those pairs of electrodes that provide the bestECG signals, in terms of signal level, noise, phase discrimination, orany other criteria. As with the previously described embodiments ofFIGS. 1A-E, the plurality of ECG sensing electrodes 10 may be deployedabout the body of the patient in pairs of generally opposed electrodes(e.g., ECG sensing electrodes 10 a, 10 b), or simply spaced about thetorso of the patient. Although not depicted in FIG. 1F, the plurality ofECG sensing electrodes 10 will typically include ECG sensing electrodespositioned on side of the patient's torso, as well as the patient'sfront and back. It should be appreciated that the embodiments of FIGS.1A-E may additionally include one or more ECG sensing electrodes (ordriven ground electrodes) that are directly attached to the body of thepatient, in addition to those that are integrated into the garment 20a-20 c.

FIG. 1G illustrates a plan view of electrode system 100 of FIGS. 1A-1E.As shown, the plurality of ECG sensing electrodes 10 are disposed atdifferent axial positions about the body of the patient, although theyneed not be deployed in a single plane. Indeed, in at least oneembodiment, the plurality of ECG sensing electrodes are not allco-located in a single plane so that pairs of electrodes correspondingto different planes may be selected. Further, although the plurality ofECG sensing electrodes 10 are shown as being deployed in generallydiametrically opposed pairs of electrodes (e.g., electrodes 10 a and 10b, electrodes 10 c and 10 d), it should be appreciated that the presentinvention is not so limited. In the embodiment depicted in FIG. 1G, theplurality of ECG sensing electrodes 10 includes 16 ECG sensingelectrodes, with each ECG sensing electrode being spaced apart from anadjacent ECG sensing electrode by approximately 22.5°. In anotherembodiment, the plurality of ECG sensing electrodes 10 includes 12sensing electrodes, with each ECG sensing electrode being spaced apartfrom an adjacent ECG electrode by approximately 30°, and in a furtherembodiment, the plurality of ECG sensing electrodes includes 18 sensingelectrodes spaced approximately 20° apart. It should be appreciated thatmore or fewer ECG sensing electrodes may be provided, for example, asfew as three, and that some or all of the ECG sensing electrodes may belocated outside of a horizontal plane intersecting the heart of thepatient.

Advantageously, the use of multiple electrodes permits differentpairings of ECG sensing electrodes to be selected, where that selectionprovides a better or more desirable ECG signal, in terms of signallevel, noise immunity, phase difference, cardiac arrhythmia detectionspecificity, or any other criteria. For example, ECG sensing electrode10 a could be paired with either of ECG sensing electrodes 10 k or 10 j,rather than with ECG sensing electrode 10 b, where such a pairingresulted in a better ECG signal level, better noise immunity, or amaximum phase discrimination, or where it was determined that ECGsensing electrode 10 b had fallen off or has poor contact with the bodyof the patient. Different pairings of ECG sensing electrodes having asimilar phase difference, or representing different phase differencesmay be selected and compared to one another. For example, ECG sensingelectrodes 10 g and 10 h that are spaced approximately 180° apart may bepaired and the ECG signal compared to that from ECG sensing electrodes10 c and 10 d (also spaced 180° apart), or alternatively, ECG sensingelectrodes 10 g and 10 h may be paired and the ECG signal compared tothat from ECG sensing electrodes 10 b and 10 d that are spacedapproximately 90° apart in order to screen out noise or deriveadditional information. Where the plurality of ECG sensing electrodes 10are not all located in a single plane, the pairings of ECG sensingelectrodes may be selected to correspond to different planes. It shouldbe appreciated that the different pairings of ECG sensing electrodesneed not be disjoint. For example, ECG sensing electrode 10 a may bepaired with ECG sensing electrode 10 b and the ECG signal compared tothat from ECG sensing electrodes 10 a and 10 c and/or to that from ECGsensing electrodes 10 a and 10 d.

FIG. 2A illustrates a signal acquisition circuit that may be used withembodiments of the present invention to select, from among a pluralityof ECG sensing electrodes, those pairings of electrodes that provide adesired ECG signal, in terms of signal-to-noise ratio, phasediscrimination, or any other criteria, and provide those ECG signals todownstream circuitry for further signal conditioning, processing,analysis, and/or monitoring. Advantageously, the signal acquisitioncircuit 200 a depicted in FIG. 2A may be used as a front end to theanalog to digital conversion and signal conditioning block 14 describedwith respect to the arrhythmia detection system of FIGS. 2a-2c of the'669 patent.

As shown, the signal acquisition circuit 200 a includes a selectioncircuit 210 that is electrically coupled to a differential circuit 220.Signals from each of the plurality of ECG sensing electrodes 10 a-10 pare provided to a respective input 212 of the selection circuit 210.Signals from one or more of the driven ground electrodes 12 may also beprovided to an input 212 of the selection circuit 210, such that asignal may be transmitted on the driven ground electrode 12, and thatsignal compared to the signals received on each of the plurality of ECGsensing electrodes to identify whether a particular ECG sensingelectrode may have fallen off, or to identify noise issues relating to aparticular ECG sensing electrode. The selection circuit 210 has aplurality of outputs 216 that are electrically coupled to respectiveinputs 222 of the differential circuit 220. In operation, the selectioncircuit 210 operates in a manner similar to a multiple outputmultiplexer, and includes a plurality of control inputs 214 to selectsignals from different ECG sensing electrodes and/or the driven groundelectrode and provide those selected signals to the inputs 222 of thedifferential circuit 220. It should be appreciated that rather than asingle selection circuit, a plurality of conventional single outputmultiplexers may be used to achieve the same functionality.

The differential circuit 220 includes a plurality of analog differentialinstrumentation amplifiers 220 a, 220 b, . . . 220 n, to receive thesignals provided by different pairings of the ECG sensing electrodesand/or different pairings of a respective ECG sensing electrode and adriven ground electrode and provide a respective differential outputsignal 226 corresponding to the difference therebetween. Where thesignals provided to a respective amplifier 220 a, 220 b, . . . 220 ncorrespond to signals provided by different ECG sensing electrodes, adifferential ECG signal is provided. This differential analog ECG signalmay then be digitally converted and conditioned by an analog-to-digitalconversion and signal conditioning block of an arrhythmia detectionsystem, such as that described with respect to FIGS. 2a-2c of the '669patent, prior to further analysis and/or monitoring by an arrhythmiamonitoring and/or treatment system, such as a wearable defibrillator.

FIG. 2B illustrates an alternative signal acquisition circuit that maybe used with embodiments of the present invention to select, from amonga plurality of ECG sensing electrodes, those pairings of electrodes thatprovide a desired ECG signal, in terms of signal-to-noise ratio, phasediscrimination, or any other criteria, and provide those ECG signals todownstream circuitry for further signal conditioning, processing,analysis, and/or monitoring. The signal acquisition circuit 200 b issubstantially similar to that of signal acquisition circuit 200 adescribed immediately above with respect to FIG. 2A, and thus, only theselection circuit 210 is shown in FIG. 2B, and the other portions ofcircuit 200 a, such as the differential circuit 220, are not depicted.However, in addition to being able to select any pairings of ECG sensingelectrodes, the signal acquisition circuit 200 b additionally allows anyone of the plurality of ECG sensing electrodes 10 a-10 p to be used as adriven ground electrode.

As known to those skilled in the art of signal processing, a drivenground electrode is frequently used to eliminate noise that may becommon to many or all sensors, such as ECG sensing electrodes 10 a-10 p.Noise signals present on some or all of the sensors, such as the ECGsensing electrodes are summed, then inverted, and then injected into thedriven ground circuitry. Where the sensors are ECG sensing electrodesthat are attached to the body of a patient, the inverted signal may beactively driven onto the body of the patient where it is picked up bythe ECG sensing electrodes, effectively cancelling out the noise thatwould normally be detected.

However, in a wearable medical device, such as the wearable medicaldevice described with respect to FIGS. 1A-F, there may be instanceswhere the driven ground electrode 12 that is used to transmit the drivenground signal to the body of the patient may have fallen off, lostcontact with the body of the patient, or simply not be workingappropriately. Indeed, even where the driven ground electrode 12 is ingood contact with the body of the patient and working properly, thedriven ground electrode may simply be located in a sub-optimal position.Where any of these conditions exist, one or more of the plurality of ECGsensing electrodes 10 a-10 p may be used as a driven ground electrode.This aspect of the present invention in now described in more detailwith respect to FIG. 2B.

As shown in FIG. 2B, a plurality of signal pads 230 can be provided witheach respective signal pad 230 a-230 p being electrically coupled to adriven ground circuit (not shown). A plurality of switches 232 areprovided, with each respective switch 232 a-p being electrically coupledbetween a respective ECG sensing electrode 10 a-p and a respective input212 a-p of the selection circuit 210. Each switch 232 a-p is capable ofbeing in one of two positions. In a first position, the switch 232electrically couples a respective ECG sensing electrode 10 a-p to arespective input 212 a-p of the selection circuit. In the secondposition, the switch 232 electrically couples the respective electrode10 a-p to a respective signal pad 230 a-p that is electrically coupledto the driven ground circuit. For example, as illustrated in FIG. 2B,switch 232 a is in a position such that the signal pad 230 a iselectrically coupled to ECG sensing electrode 10 a, whereas each ofswitches 232 b, 232 c, . . . 232 p are in a position such that ECGsensing electrodes 10 b-10 p are respectively electrically coupled to arespective input 212 b-212 p of the selection circuit 210. In thisconfiguration, ECG sensing electrode 10 a may be used as a driven groundelectrode, where that use provides a better signal on others of theplurality of ECG sensing electrodes, where another driven groundelectrode has lost contact or has poor contact with the body of thepatient, or for any other reason.

FIG. 2C illustrates a further alternative signal acquisition circuitthat may be used with embodiments of the present invention to selectpairings of ECG sensing electrodes in a manner similar to that describedabove with respect to FIGS. 2A and 2B, and to allow any one of theplurality of ECG sensing electrodes to be used as a driven groundelectrode in a manner similar to that described above with respect toFIG. 2B. The signal acquisition circuit 200 c is substantially similarto that of signal acquisition circuit 200 b described immediately above,and therefore, only the differences are described.

As in the signal acquisition circuit 200 b, a plurality of signal pads230 are provided with each respective signal pad 230 a-230 p beingelectrically coupled to a driven ground circuit (not shown). A pluralityof switches 232 is also provided. Each respective switch 232 a-p of theplurality of switches 232 is electrically coupled to a respective signalpad 230 a-p of the plurality of signal pads 230, which in turn, iselectrically coupled to a driven ground circuit (not shown). Each switch232 a-p is capable of being in one of two positions, opened and closed.In the open position, the driven ground signal on a respective signalpad 230 a-p is an open circuit, and in the closed position, the switch232 electrically couples a respective ECG sensing electrode 10 a-p to arespective signal pad 230 a-p. For example, as illustrated in FIG. 2C,switch 232 a and each of switches 232 c-232 p is in an open position,and switch 232 b is in a closed position, such that the signal pad 230 bis electrically coupled to ECG sensing electrode 10 b. This embodimentrelies on the fact that the selection circuit 210 generally will have arelatively high input impedance, such that each of the inputs 212 mayremain connected to an ECG sensing electrode 10 while that ECG sensingelectrode is electrically coupled to the driven ground circuit, as thedriven ground circuit will typically have a relative low outputimpedance. In this configuration, ECG sensing electrode 10 b may be usedas a driven ground electrode, where that use provides a better signal onothers of the plurality of ECG sensing electrodes, where another drivenground electrode has lost contact or has poor contact with the body ofthe patient, or for any other reason.

It should be appreciated that in the embodiments of FIGS. 2B and 2Cdescribed above, more than one driven ground circuit may be provided.For example, ECG sensing electrode 10 a could be used as a driven groundelectrode for use with ECG sensing electrodes 10 c, 10 d, 10 o, 10 n, 10e, 10 h, 10 i, and 10 l (see FIG. 1G), and ECG sensing electrode 10 bcould be used as a driven ground electrode for use with ECG sensingelectrodes 10 m, 10 p, 10 g, 10 f, 10 k, and 10 j.

FIG. 3 illustrates a signal acquisition circuit according to anotherembodiment of the present invention that may be used to acquire signalsfrom different pairings of ECG sensing electrodes, or from differentpairings of ECG sensing electrodes and a driven ground electrode, andprovide those signals to downstream circuitry, such as an A/D conversionand signal conditioning block 330 of an arrhythmia monitoring and/ortreatment system. As shown in FIG. 3, the signal acquisition circuit 300includes a plurality of analog differential instrumentation amplifiers320 a-320 n, 321 a-321 n-1, . . . 329 a that are each configured toreceive signals from different pairings of ECG sensing electrodes. Forexample, a first grouping of amplifiers 320 a-320 n may be configured torespectively pair each of ECG sensing electrodes 10 b-10 p (FIG. 1G)with ECG sensing electrode 10 a, a second grouping of amplifiers 321a-321 n-1 may be configured to respectively pair each of ECG sensingelectrodes 10 c-10 p with ECG sensing electrode 10 b, etc. Although notshown, each grouping of amplifiers may also compare a signal from arespective ECG sensing electrode with a driven ground electrode, or witheach of a number of driven ground electrodes. In contrast to the signalacquisition circuit 200 of FIGS. 2A-C, individual signals from each ofthe different pairings of ECG sensing electrodes are provided directlyto the inputs of a respective amplifier. As such, this embodiment mayavoid noise or signal degradation caused by the selection circuit 210being located prior to the differential circuit 220. Such selectioncircuitry may instead be provided after an analog-to-digital conversionand signal conditioning block, such as the analog-to-digital conversionand signal conditioning block 14 described with respect to FIGS. 2a-2cof the '669 patent. For example, as shown in FIG. 3, the analogdifferential output signals provided by each respective amplifier 320a-320 n, 321 a-321 n-1, 329 a may be provided to a respective input 332of an A/D conversion and signal conditioning block 330 that digitizesand conditions the analog differential output signals. The digitized andconditioned signals provided on a respective output 334 of the A/Dconversion and signal conditioning block 330 may then be provided to arespective input 342 of an output selection circuit 340 that operates ina manner similar to a multiple output multiplexer. Responsive to controlsignals provided to control inputs 344 of the selection circuit 340, theselection circuit selects, from among the plurality of digitized andconditioned signals, which of those digitized and conditioned signals toprovide to a respective output 346 of the selection circuit formonitoring and/or analysis.

In accordance with an aspect of the present invention, each of thedifferent pairings of ECG sensing electrodes 10 may be selected andtheir signals analyzed to identify those pairings of ECG sensingelectrodes that provide a desired ECG signal, in terms ofsignal-to-noise ratio, phase discrimination, or any other criteria.Those pairings of ECG sensing electrodes providing the highestsignal-to-noise ratio, a particular phase discrimination or a maximumphase discrimination, or those pairings of electrodes corresponding toparticular planes may then be selected to provide those signals to acardiac monitor, or to an arrhythmia detection system, such as thatillustrated in FIGS. 2a-c of the '669 patent. For example, referring toFIG. 1G, if it were determined that the pairing of ECG sensingelectrodes 10 a and 10 j, and 10 c and 10 d provided the highest qualitysignal (in terms of a high signal-to-noise ratio and maximum phasediscrimination), but the pairing of ECG sensing electrodes 10 a and 10 bdid not, the signal from ECG sensing electrode 10 j would be paired withECG sensing electrode 10 a and the signals from these ECG sensingelectrodes could be analyzed with respect to the signals from ECGsensing electrodes 10 c and 10 d.

It should be appreciated that embodiments of the present inventionprovide a cardiac monitoring system and/or a cardiac monitoring andarrhythmia detection system with the ability to select, from among aplurality of electrodes, those pairings of electrodes that provide thehighest quality signal, a particular phase difference or a maximum phasediscrimination, or any other criteria. With this ability to choose ECGsensing electrodes, the analyzer of the cardiac monitoring andarrhythmia detection system can, for example, be tuned to give the bestorthogonal view and can provide more cardiac information than a singleor dual channel sensing system. The analyzer can select multipletemplates representing different phase angles between ECG sensingelectrode leads, or templates representing different planes of view ofthe patient's heart. Each electrode channel can be auto correlated(compared to itself) or cross correlated (compared with other channels)in order to screen out noise and derive additional information.

Embodiments of the present invention can also return to the best axispositions if the overall electrode system was shifted at a later time,such as when the electrodes are configured as part of a wearableelectrode belt or garment system. Because this multiple electrodeconfiguration can select the electrodes with the best quality signal,the number of alarms due to ECG noise and fall-off can be reduced.Another byproduct of a cleaner ECG signal is a reduction in falsedetections. By checking multiple electrodes, and finding that themajority are sensing the same thing, embodiments of the presentinvention can increase the confidence level of the detection algorithm.In addition, each time the electrode belt or garment is worn, theelectrodes may move to a slightly different location, resulting in achange to the ECG signal. With multiple electrode configurations, thedetection system can scan the multiple paths and select the highestquality signals. Furthermore, by providing redundancy to the sensingsystem, this multiple electrode configuration helps to improve theoverall system reliability. A fault in one or more channels can betolerated because there are other working channels. These and otheraspects of the present invention are now described with respect to FIGS.4-8.

FIG. 4 functionally illustrates a control unit, such as the control unit30 depicted in FIGS. 1A-F that may be used by a portable medical device,such as a cardiac monitor or a wearable defibrillator, in accordancewith the present invention. As shown, the control unit 30 includes atleast one processor 410, a battery 420, a data storage 412, a sensorinterface 414, a therapy delivery interface 416, and a user interface418. The battery 420 may be a rechargeable three cell 2200 mAh lithiumion battery pack that provides electrical power to the other devicecomponents. The data storage 412, the sensor interface 414, the therapydelivery interface 416, and the user interface 418 are coupled to the atleast one processor 410. The data storage 412 includes a computerreadable and writeable data storage medium configured to storenon-transitory instructions and other data, and can include bothnonvolatile storage media, such as optical or magnetic disk, ROM orflash memory, as well as volatile memory, such as RAM. The instructionsmay include executable programs or other code that can be executed bythe at least one processor 410 to perform any of the functions describedhere below.

The therapy delivery interface 416 couples one or more therapy deliverydevices, such as defibrillator therapy electrodes 12 a-c (FIG. 1E), tothe at least one processor 410. Where the control unit is used solelyfor monitoring a patient's cardiac condition, the therapy interface 416and associated defibrillation therapy electrodes may be omitted. Theuser interface 418 includes a combination of hardware and softwarecomponents that allow the control unit 30 to communicate with anexternal entity, such as a user. These components are configured toreceive information from actions such as physical movement, verbalintonation or thought processes. In addition, the components of the userinterface 418 can provide information to external entities, for example,in a manner such as described in U.S. Pat. No. 6,681,003, which isincorporated herein by reference. Examples of the components that may beemployed within the user interface 418 include keyboards, mouse devices,trackballs, microphones, electrodes, touch screens, printing devices,display screens and speakers.

The sensor interface 414 couples the at least one processor 410 to aplurality of physiological sensors, such as the plurality of ECG sensingelectrodes 10. In some embodiments, the sensor interface 414 may alsocouple the at least one processor 410 to other physiological sensors,such as activity sensors, pulse oxygen sensors, temperature sensors,respiratory rate sensors, thoracic impedance sensors, blood pressuresensors, acoustic sensors, etc. The sensor interface 414 can include asignal acquisition circuit, such as the signal acquisitions circuits 200and 300 described above with respect to FIGS. 2A-C and 3, or the signalacquisition circuits 900, 1000, and 1100 described further below withrespect to FIGS. 9-11, to select, from among the plurality of ECGsensing electrodes and/or other physiological sensors, those thatprovide a desired signal, in terms signal-to-noise ratio, phasediscrimination, or any other criteria.

Although not illustrated in FIG. 4, the control unit 30 may includeadditional components and or interfaces, such as a communication networkinterface (wired and/or wireless), and the at least one processor 410may include a power conserving processor arrangement such as describedin co-pending application Ser. No. 12/833,096, entitled SYSTEM ANDMETHOD FOR CONSERVING POWER IN A MEDICAL DEVICE, filed Jul. 9, 2010(hereinafter the “'096 application”), and incorporated by referenceherein in its entirety. For example, as described in the '096application, the at least one processor 410 may include a generalpurpose processor, such as an Intel® PXA270 processor that is coupled toa critical purpose processor, such as a Freescale™ DSP56311 DigitalSignal Processor (DSP). The general purpose processor can be configuredto perform non-critical functions that do not require real timeprocessing, such as interfacing with the communication network interfaceand the user interface, while the critical purpose processor isconfigured to perform critical functions that require real timeprocessing, such as the sampling and analysis of ECG information, thecharging of the capacitors to a particular voltage, and the generationand/or delivery of therapeutic defibrillating pulses. It should beappreciated that in some embodiments, the functionality of the at leastone processor may be implemented in a Field Programmable Gate Array(FPGA), one or more Programmable Logic Devices (PLDs), a Complex PLD(CPLD), or custom Application Specific Integrated Circuit (ASIC).

FIGS. 5-8 illustrate a number of different processes that may beperformed by the at least one processor 410 of the control unit 30 toimprove the monitoring and analysis of cardiac activity, to improve thedetection of cardiac abnormalities, and to reduce the number of falsedetections and fall-off alarms in accordance with embodiments of thepresent invention.

FIG. 5 illustrates a selection process that may be executed by the atleast one processor 410 of the control unit 30 to select, from among aplurality of ECG sensing electrodes 10, those providing a highestquality ECG signal, in terms of signal-to-noise ratio and maximum phasediscrimination, in accordance with one embodiment of the presentinvention. In act 510, the at least one processor 410 selects a pair ofECG sensing electrodes to monitor. As discussed previously, this may beperformed by the at least one processor sending appropriate controlsignals to selection circuit 210, 340.

In act 520, the at least one processor analyzes the ECG signal obtainedfrom the selected pair of ECG sensing electrodes and records informationidentifying the selected pair of ECG sensing electrodes and a metricindicative of the quality of the ECG signal provided therefrom. Althougha number of different criteria may be used to identify the quality ofthe ECG signal, in one embodiment, those ECG signals having a highestsignal-to-noise ratio and a maximum phase discrimination are assigned ahigher quality metric than those pairings that do not.

In act 530, the at least one processor determines whether each of thepossible pairings of ECG sensing electrodes have been selected andanalyzed. Where it is determined that all the possible pairings of ECGsensing electrodes have been selected and analyzed, the process proceedsto act 540. Alternatively, where it is determined that fewer than all ofthe possible pairs of ECG sensing electrodes have been selected andanalyzed, the process returns to act 510, where a next sensor pairing isselected. Acts 510 through 530 are then performed for each of thepossible pairings of ECG sensing electrodes.

In act 540, the at least one processor selects, from among the pluralityof different pairings of ECG sensing electrodes, those pairs of ECGsensing electrodes having the highest quality metric. It should beappreciated that the number of different pairings of ECG sensingelectrodes that are selected in act 540 will depend on the number ofdifferent channels provided at the output 226 of the differentialcircuit 220 (FIG. 2A) or at the output 346 of the selection circuit 340of FIG. 3. In general, a minimum of two channels would be selected inact 540, and in most implementations, at least four different channelswould be selected. In some embodiments, the number of channels providedmay correspond to each unique pairing of electrodes.

In act 550, the at least one processor monitors and analyzes the ECGsignals provided by the selected pairings of ECG sensing electrodes. Theact of monitoring and analyzing the ECG signals provided by the selectedECG sensor pairs (i.e., act 550) may continue until the terminated byremoval and/or power down of the wearable medical device.

In accordance with one embodiment of the present invention, theselection process described with respect to FIG. 5 may be performed eachtime the electrode system 100 is powered on to account for any potentialrepositioning of the plurality of electrodes 10 on the body of thepatient. Thus, for example, when a garment 20 a-20 c incorporating anelectrode system 100 is removed from the body of the patient to allowthe patient to shower, for service, or for any other reason, and thenreturned to a position on the patient's body, the positioning of some orall of the ECG sensing electrodes may change from their prior position.By re-executing the selection process of FIG. 5, the electrode systemmay select those pairings of ECG sensing electrodes that provide thehighest quality ECG signals, irrespective of whether those pairings ofECG electrodes are the same, or different from those selectedpreviously. In certain implementations, an initial pairing of ECGsensing electrodes may be based upon those that were previously selectedin act 540. For example, in response to a garment incorporating anelectrode system 100 being removed from and returned to the body of apatient, the electrode system may initially select pairings of ECGsensing electrodes based upon those that were selected prior to removalof the garment in act 540. That initial selection may then be confirmedby re-executing the selection process of FIG. 5.

It should be appreciated that the selection process described withrespect to FIG. 5 may be re-executed, either at periodic intervals(e.g., every half hour), or in response to another sensor, such as anactivity sensor, indicating strenuous physical activity, to ensure theoptimal pairings of ECG sensing electrodes are selected. By re-executingthe selection process, either periodically, or in response to detectedphysical activity, embodiments of the present invention can ensure thatthose pairings of ECG sensing electrodes providing the highest qualityECG are identified and used for monitoring and analysis.

Although the selection process of FIG. 5 was described as selectingthose pairs of ECG sensing electrodes providing the highest quality ECGsignal, in terms of signal-to-noise ratio and maximum phasediscrimination, it should be appreciated that other criteria may beused. For example, the process described with respect to FIG. 5 may bemodified to include an act of selecting a desired template prior to act510. The desired template may, for example, reflect different phaseangles between ECG sensing electrodes that are desired to be monitored.The acts 510 and 520 of selecting and analyzing different ECG sensingelectrode pairings could thus select, from among the plurality of ECGsensing electrodes, those pairs of ECG sensing electrodes that providethe highest signal-to-noise ratio from among those pairings that meetthe desired phase angle(s) of the template. It should be appreciatedthat other criterion, other than phase angle, may be reflected in atemplate, and that multiple templates may be provided and/or selected.For example, one template may correspond to different pairings of ECGsensing electrodes that correspond to different planes intersecting thepatient's heart, while another template may correspond to differentpairing of ECG sensing electrodes that are all co-located in the sameplane.

FIG. 6 illustrates a noise/fall-off detection process that may beexecuted by the at least one processor 410 of the control unit 30 (FIG.4) in accordance with an aspect of the present invention to improve thequality of monitoring and analysis of ECG signals and/or to reduce thenumber of fall-off alarms. In act 610 the at least one processormonitors and analyzes selected ECG signals from different pairings ofECG sensing electrodes. The pairings of ECG sensing electrodes that aremonitored and analyzed in act 610 may have been previously selectedbased upon a selection process such as that described with respect toFIG. 5. In act 620, the at least one processor makes a determination asto whether there is noise in the ECG signal of a selected pairing of ECGsensing electrodes, or whether there has been a fall-off or at leastpartial loss of contact with the body of the patient by a selectedpairing of ECG sensing electrodes. Where it is determined in act 620that there is no appreciable noise or a diminished signal or a lack ofsignal on any of the selected pairings of ECG sensing electrodes, the atleast one processor returns to act 610 and continues monitoring theselected ECG signals. Alternatively, where it is determined that thereis appreciable noise or a diminished signal or lack of signal from oneof the selected pairings of ECG sensing electrodes, the process proceedsto act 630.

In act 630 the at least one processor 410 selects a new pairing of ECGsensing electrodes to replace the pairing in which increased noise, or adiminished ECG signal was detected. Act 630 may be performed in a mannersimilar to the selection process described with respect to FIG. 5. Inact 630, each of the possible pairings of ECG sensing electrodes may bere-evaluated to select those pairings of ECG electrodes to be monitored.Alternatively, those selected pairings of ECG sensing electrodes inwhich noise or fall-off was not detected may be retained as selectedpairings, and the remaining ECG sensing electrodes evaluated to identifyand select a pairing of ECG sensing electrodes to replace the pairing inwhich noise or fall-off was detected. In response to the selection of anew pairing of ECG sensing electrodes, or a number of new pairings, theprocess returns to monitoring an analyzing ECG signals in act 610.

Although not shown in FIG. 6, in response to the detection of noise orfall-off in a selected pairing of ECG sensing electrodes, the at leastone processor 410 may conduct additional tests on the selected pairing.For example, the at least one processor may pair each ECG sensingelectrode of the selected pair with a driven ground electrode toidentify which of the ECG sensing electrodes of the selected pair mayhave a noise issue or may have at least partially lost contact with thebody of the patient. The at least one processor 410 may also send amessage to the user of portable medical device (or a bystander) via theuser interface 418 to notify the user that one or more of the ECGsensing electrodes of the selected pairing may have a noise issue or mayhave at least partially lost contact with the body of the patient, andmay further request the user to reposition the ECG sensing electrodes ofthe selected pairing.

FIG. 7 illustrates monitoring and analysis routine that may be executedby the at least one processor 410 of the control unit 30 to improve thedetection of cardiac arrhythmias and reduce the number of falsedetections. In act 710 the at least one processor monitors and analyzesselected ECG signals from different pairings of ECG sensing electrodes.The pairings of ECG sensing electrodes that are monitored and analyzedin act 710 may have been previously selected based upon a selectionprocess such as that described with respect to FIG. 5. In act 720 adetermination is made as to whether a cardiac arrhythmia has beendetected. Where it is determined in act 720 that a cardiac arrhythmia,such as ventricular tachycardia or ventricular fibrillation, has notbeen detected, the process returns to act 710 and continues to monitorand analyze the selected ECG signals. Alternatively, where it isdetermined in act 720 that a cardiac arrhythmia has been detected, theat least one processor proceeds to act 730 wherein the at least oneprocessor sets a flag or indicator identifying that a cardiac arrhythmiahas been detected, with the at least one processor proceeding to act740.

In act 740, the at least one processor 410 selects a different oradditional pairing of ECG sensing electrodes to monitor, to identifywhether the determined arrhythmia is also present in the ECG signalsfrom other pairings of ECG sensing electrodes. The additional ordifferent pairings of ECG sensing electrodes may be based upon theselection process described previously with respect to FIG. 5. Forexample, the additional or different pairings of ECG sensing electrodesthat are selected in act 740 may be one or more of those pairings thatprovides the next highest signal quality level other than those thatwere selected in act 540 of FIG. 5. In act 750, the at least oneprocessor continues to monitor and analyze the selected ECG signals,including those from additional or different pairings of ECG sensingelectrodes selected in act 740.

In act 760, the at least one processor 410 again determines whether acardiac arrhythmia has been detected, based upon the ECG signalsmonitored in act 750. Where it is determined that a cardiac arrhythmiahas not been detected in the different or additional pairings, the atleast one processor may simply return to act 750 and continue to monitorthe selected ECG signals. However, where it is determined in act 760that a cardiac arrhythmia, such as ventricular tachycardia orventricular fibrillation has been detected, the at least one processormay proceed to act 770. In act 770, in response to detecting that thecardiac arrhythmia is still present, or is also present on the selectedadditional or different pairings of ECG sensing electrodes, the at leastone processor increases a confidence level of the indicator or flag setin act 730. Although not depicted in FIG. 7, in response to theconfidence level being above a certain threshold, and the cardiacarrhythmia being a type of cardiac arrhythmia for which defibrillationis an appropriate treatment, the at least one processor 410 may executeone or more instructions that result in defibrillation being applied tothe body of the patient via the therapy delivery interface 416. Inaccordance with this aspect of the present invention, by examining otherpairings of ECG sensing electrodes in response to a detected cardiacarrhythmia, the detection specificity of cardiac arrhythmias may beincreased and the number of false detections of cardiac malfunction maybe reduced.

FIG. 8 illustrates a monitoring and analysis routine in accordance withanother embodiment of the present invention that may be executed by theat least one processor 410 of the control unit 30 to improve themonitoring and analysis of cardiac activity. In act 810 the at least oneprocessor monitors and analyzes selected ECG signals from differentpairings of ECG sensing electrodes. The pairings of ECG sensingelectrodes that are monitored and analyzed in act 810 may have beenpreviously selected based upon a selection process such as thatdescribed with respect to FIG. 5, or they may have been selected forother reasons. For example, the pairings of ECG electrodes may notprovide the highest quality ECG signal of all of the ECG sensor pairs,but may correspond to a particular plane or planes, or to a particularposition relative to the heart.

In act 820, the at least one processor 410 monitors and analyzes the ECGsignals provided by the selected pairings of ECG sensing electrodes. Inact 830 a determination is made as to whether to select new pairs of ECGsensing electrodes to monitor. The determination as to whether to selectnew pairs of ECG sensing electrodes may be based upon a number ofdifferent criteria, including the number of channels that are capable ofbeing monitored and analyzed at a time, the type of information that issought, the stage of the cardiac cycle (e.g., the diastolic stage, orthe systolic stage), the position of the ECG sensing electrodes relativeto the heart and/or the stage of depolarization or repolarization of theheart (e.g., as indicated by PQRST waveform of the ECG signals), etc.For example, where the control unit 30 is capable of simultaneouslymonitoring three different channels and the plurality of ECG sensingelectrodes 10 includes 12 ECG sensing electrodes, three pairings of ECGsensing electrodes (including six distinct ECG sensing electrodes) maybe monitored and analyzed during a first time interval, and theremaining three pairings of ECG sensing electrodes that were notmonitored and analyzed during the first interval may be monitored andanalyzed during a second and subsequent time interval. Alternatively,where the control unit is capable of simultaneously monitoring threedifferent channels and the plurality of ECG sensing electrodes 10includes 16 ECG sensing electrodes (as shown in FIG. 1G), threedifferent pairings of ECG sensing electrodes including ECG sensingelectrode pairs 10 o-10 p, 10 c-10 d, 10 m-10 n may be monitored andanalyzed during a first time interval, three different pairings of ECGsensing electrodes including ECG sensing electrode pairs 10 g-10 h, 10k-10 l, and 10 b-10 a may be monitored during a second interval, andthree different pairs of ECG sensing electrodes including ECG sensingelectrodes pairs 10 j-10 i, 10 f-10 e, and 10 o-10 p may be monitoredand analyzed during a third time interval. In this manner, the selectedpairings of ECG electrodes may sweep about the circumference of theheart. It should be appreciated that where the number of channels thatcan be simultaneously monitored by the control unit 30 are sufficient tomonitor all pairings of ECG sensing electrodes, or all unique pairingsof ECG sensing electrodes, then all such pairings may be monitoredsimultaneously.

Accordingly, in act 830, where it is determined that a new or differentpairing of ECG sensing electrodes are to be monitored and analyzed, themonitoring and analysis routine returns to act 810 wherein those new ordifferent pairings of ECG sensing electrodes are selected (act 810) andmonitored and analyzed (act 820). Alternatively, where it is determinedin act 830 that a new or different pairing of ECG sensing electrodes isnot desired, the routine returns to act 820 and continues monitoring thepairings of previously selected ECG sensing electrodes.

FIG. 9 illustrates an alternative signal acquisition circuit that may beused with embodiments of the present invention to select, from among aplurality of ECG sensing electrodes, those pairing of electrodes thatprovide a desired ECG signal, in terms of signal-to-noise ratio, phasediscrimination, or any other criteria, and provide those ECG signals todownstream circuitry for further signal conditioning, processing,analysis, and/or monitoring. In contrast to the embodiments describedpreviously with respect to FIGS. 2A-C and 3, the signal acquisitioncircuit 900 does not include any differential amplifiers but insteadgenerates differential ECG signals corresponding to selected pairings ofECG sensing electrodes in software executed by a processor, such as theat least one processor 410 described previously with respect to FIG. 4.

As shown, the signal acquisition circuit 900 includes an analogmultiplexor 910 and an analog-to-digital (A/D) converter 920. Signalsfrom each of the plurality of ECG sensing electrodes 10 a-10 p areprovided to a respective input of a plurality of inputs 912 of theanalog multiplexor 910. The analog multiplexor has an output 916 that iselectrically coupled to an input 922 of the A/D converter 920. Theanalog multiplexor 910 includes a plurality of control inputs 914 toselect which one of the plurality of signals received from a respectiveECG sensing electrode 10 a-10 p is provided to the input 922 of the ofthe A/D converter 920. The A/D converter 920 receives the selectedsignal from the selected one of the plurality of ECG sensing electrodesand converts that analog ECG sensor signal to a digital signal. Toensure adequate resolution for the processing of the digitized signalsthat is performed by the at least one processor 410, the A/D converter920 may be a 24 bit A/D converter, although an A/D converter with fewerbits may be used. In general, the sampling rate of the A/D converter 920should be at least N times the desired sampling rate of the ECG signal,where N is the number of ECG sensing electrodes that are desired to bemonitored. For example, where it is desired to monitor ECG signalsprovided by each of three pairs of ECG sensing electrodes at a samplingrate of 400 Hz, the A/D converter 920 should have a sampling rate inexcess of 2.4 KHz. It should be appreciated that higher sampling ratesmay of course be used.

Although not shown in FIG. 9, each of the signals from a respective ECGsensing electrode 10-10 p may first be buffered, filtered, and/oramplified prior to being received at a respective input of the analogmultiplexor 910. For example, each of the signals received from arespective one of the plurality of ECG sensing electrodes 10 a-p may beprovided to the input of a high impedance buffer so that the analogmultiplexor and the A/D converter to do not load down the respective ECGsensing electrode. The output of a respective buffer may be low-passfiltered (i.e., anti-aliased) to ensure that any frequency components ofthe signal are below the Nyquist frequency of the A/D converter 920, andthe filtered signal provided to a low-noise and low to moderate gainamplifier to amplify the signal before that signal is provided to arespective input of the analog multiplexor 910. As would be appreciatedby one skilled in the art, the combination of buffering, filtering,and/or amplifying the signal received from each of the plurality of ECGsensing electrodes may be performed in multiple and distinct stages(e.g., a high impedance buffer stage followed by a filtering stage andone or more amplification stages), or some of the stages, such as thebuffering and amplification stages may be performed in a single stage(e.g., a high impedance low-noise amplifier with low to moderate gain).In some embodiments, the amplification stage may be programmable by theat least one processor 410.

In accordance with one embodiment, the analog multiplexer 910 may be aconventional analog multiplexer, available from companies such as AnalogDevices, Inc. of Norwood Mass., in which control signals received on thecontrol inputs of the analog multiplexer select which one of the signalsreceived on a respective input of the multiplexer is provided to theoutput. The A/D converter 920 converts the received signal to a digitalsignal and provides the converted digital signal to the at least oneprocessor 410. The at least one processor is configured to control themultiplexor 910 and the A/D converter 920 to sample and convert each ofthe signals received from a respective ECG sensing electrode over adifferent time interval and provide the converted signals to the atleast one processor 410. Dependent upon which of the plurality of ECGsensing electrodes 10 a-p are selected to be paired with one another,the at least one processor 410 takes the two selected digital signals,inverts one of them, and digitally sums the signals, effectivelyperforming the same functionality as the differential instrumentationamplifiers described with respect to FIGS. 2A-C and 3 above. Theselection, inversion, and summing of selected pairs of digital signalsmay be performed for any pairing of ECG sensing electrodes. Thedigitally summed signals may then be processed to monitor the patient'sECG signals, to detect any arrhythmic cardiac condition, or both. Itshould be appreciated that which of the pairs of ECG sensing electrodesto pair and monitor may be performed in software by the at least oneprocessor in a manner similar to that shown in FIG. 5. Each of thedigitized signals may be compared to one another for maximum phasedifference, or a specific phase difference, or for any other criterion.Those pairings of ECG sensing electrodes may then be selected andmonitored and analyzed in the manner described above.

In accordance with an alternate embodiment, the analog multiplexer 910may be an analog sample-and-hold multiplexer that is capable ofsimultaneously sampling signals received from each of the plurality ofECG sensing electrodes over a first time period, and then providing eachof the plurality of sampled signals to the A/D converter 920 duringsubsequent time periods. In this embodiment, the at least one processor410 is configured to control the analog multiplexer 910 and the A/Dconverter 920 to sample and hold the signals received from each of theplurality of ECG sensing electrodes 10 a-p over a first time period, andprovide each, or selected ones, of the sampled signals to the A/Dconverter 920 to be converted to digital signals and provided to the atleast one processor over subsequent time periods. As in the embodimentdescribed above, dependent upon which of the plurality of ECG sensingelectrodes 10 a-p are selected to be paired with one another, the atleast one processor 410 takes the two selected digital signals, invertsone of them, and digitally sums the signals, effectively performing thesame functionality as the differential instrumentation amplifiersdescribed with respect to FIGS. 2A-C and 3 above. The selection,inversion, and summing of selected pairs of digital signals may beperformed for any pairing of ECG sensing electrodes. The digitallysummed signals may then be processed to monitor the patient's ECGsignals and/or to detect any arrhythmic cardiac condition.

FIG. 10 illustrates a further alternative signal acquisition circuitthat may be used with embodiments of the present invention to select,from among a plurality of ECG sensing electrodes, those pairing ofelectrodes that provide a desired ECG signal, in terms ofsignal-to-noise ratio, phase discrimination, or any other criteria, andprovide those ECG signals to downstream circuitry for further signalconditioning, processing, analysis, and/or monitoring. In contrast tothe embodiments described previously with respect to FIGS. 2A-C and 3,and in a manner similar to the embodiment of FIG. 9, the signalacquisition circuit 1000 does not include any differential amplifiers,but instead generates differential ECG signals corresponding to selectedpairings of ECG sensing electrodes in software executed by a processor,such as the at least one processor 410 described previously with respectto FIG. 4.

As shown, the signal acquisition circuit 1000 includes a plurality ofanalog-to-digital (A/D) converters 1010 a-p. Each of the plurality ofA/D converters 1010 a-p is configured to receive a signal from arespective one of the plurality of ECG sensing electrodes 10 a-p, forexample, with a first A/D converter 1010 a receiving a signal from ECGsensing electrode 10 a, A/D converter 1010 b receiving a signal from ECGsensing electrode 10 b, etc. Each respective A/D converter 1010 a-pconverts the signal to a digital signal and provides the converteddigital signal to the at least one processor 410 over a communicationlink 1020, such as a serial or parallel bus. Although not shown in FIG.10, each of the signals from a respective ECG sensing electrode 10-10 pmay first be buffered, filtered, and/or amplified prior to beingreceived at a respective input of a respective A/D converter 1010 a-p ina manner similar to that described above with respect to FIG. 9 so thatthe A/D converter does not load down the respective ECG sensingelectrode, and to ensure that any frequency components of the receivedsignals are below the Nyquist frequency of a respective A/D converter1010 a-p.

To ensure adequate resolution for the processing performed by the atleast one processor 410, each of the plurality of A/D converters 1010a-p may be a 24 bit A/D converter, although an A/D converter with fewerbits may be used. In contrast to the embodiment described above withrespect to FIG. 9, each of plurality of A/D converters 1010 a-p of thisembodiment need not have a sampling rate that is N times the desiredsampling rate of the ECG signal, where N is the number of ECG sensingelectrodes that are desired to be monitored, because each of the signalsreceived from a respective ECG sensing electrode may be sampled inparallel. For example, where it is desired to monitor ECG signalsprovided by each of three pairs of ECG sensing electrodes at a samplingrate of 400 Hz, each of the plurality of A/D converters may have asampling rate of 400 Hz, thereby allowing the use of less costly A/Dconverters. Of course, it should be appreciated that higher samplingrates may be used. In accordance with this embodiment, the at least oneprocessor 410 may send a control signal to each of the plurality of A/Dconverters 1010 a-p to sample a respective signal at substantially thesame period of time, and then send the sampled and converted digitalsignal to the processor at a subsequent time. Dependent upon which ofthe plurality of ECG sensing electrodes are selected to be paired withone another, the at least one processor takes the two selected digitalsignals, inverts one of them, and digitally sums the signals,effectively performing the same functionality as the differentialinstrumentation amplifiers described with respect to FIGS. 2A-C and 3above. The selection, inversion, and summing of selected pairs ofdigital signals may again be performed for any pairing of ECG sensingelectrodes. The digitally summed signals may then be processed tomonitor the patient's ECG signals and/or to detect any arrhythmiccardiac condition.

FIG. 11 illustrates yet a further alternative signal acquisition circuitthat may be used with embodiments of the present invention to select,from among a plurality of ECG sensing electrodes, those pairing ofelectrodes that provide a desired ECG signal, in terms ofsignal-to-noise ratio, phase discrimination, or any other criteria, andprovide those ECG signals to downstream circuitry for further signalconditioning, processing, analysis, and/or monitoring. This embodimentis similar to the embodiment described above with respect to FIGS. 9 and10 in that it again does not include any differential amplifiers, butinstead generates differential ECG signals corresponding to selectedpairings of ECG sensing electrodes in software executed by a processor,such as the at least one processor 410 described previously with respectto FIG. 4.

As in the embodiment described above with respect to FIG. 10, the signalacquisition circuit 1100 shown in FIG. 11 again includes a plurality ofanalog-to-digital (A/D) converters 1110 a-p, each of which is configuredto receive a signal from a respective one of the plurality of ECGsensing electrodes 10 a-p. Although not shown in FIG. 11, each of thesignals from a respective ECG sensing electrode 10-10 p may first bebuffered, filtered, and/or amplified prior to being received at arespective input of a respective A/D converter 1110 a-p in a mannersimilar to that described above with respect to FIGS. 9 and 10 so thatthe A/D converter does not load down the respective ECG sensingelectrode, and to ensure that any frequency components of the receivedsignal are below the Nyquist frequency of a respective A/D converter1110 a-p.

In contrast to the embodiment of FIG. 10 in which the plurality of A/Dconverters 1010 a-p are arranged in parallel, each of the plurality ofA/D converters 1110 a-p of this embodiment are daisy chained (orcascaded) to one another, for example via a serial bus, such as a SPI™serial bus, a QSPI™ serial bus, or Microwire™ serial bus. Eachrespective A/D converter 1110 a-p is arranged to sample a signal from arespective one of the plurality of ECG sensing electrodes 10 a-p duringa first time interval, and convert the signal to a digital signal andprovide the converted digital signal to the next A/D converter in thechain during a subsequent time interval. The output of the last A/Dconverter 1110 p in the chain is communicatively coupled to the at leastone processor via a communication link 1120, such as a serial bus. Theoutput of the last A/D converter in the chain (e.g., A/D converter 1110p) therefore provides a multi-bit signal to the at least one processor410 with different bits in the multi-bit signal corresponding todifferent ECG sensing electrodes, for example, with a first series ofbits corresponding to the converted digital signal obtained from ECGsensing electrode 10 p, the second series of bits corresponding to theconverted digital signal obtained from ECG sensing electrode 10 o, andthe last series of bits corresponding to the converted digital signalobtained from ECG sensing electrode 10 a.

To ensure adequate resolution, each of the plurality of A/D converters1110 a-p may be a 24 bit A/D converter, although an A/D converter withfewer bits, such as 16 bits may alternatively be used. In contrast tothe embodiment described above with respect to FIG. 9, and similar tothe embodiment described above with respect to FIG. 10, each ofplurality of A/D converters 1110 a-p of this embodiment need not have asampling rate that is N times the desired sampling rate of the ECGsignal, where N is the number of ECG sensing electrodes that are desiredto be monitored, because each of the signals received from a respectiveECG sensing electrode may be sampled in parallel. Thus, where it isdesired to monitor ECG signals provided by each of three pairs of ECGsensing electrodes at a particular sampling rate, each of the pluralityof A/D converters may operate at that same sampling rate, therebyallowing the use of less costly A/D converters. However, because theplurality of A/D converters 1110 a-p are daisy chained together, therate at which the converted digital signals are communicated from oneA/D converter to the next, and then to the at least one processor 410should be at least N times the desired sampling rate of the ECG signal,where N corresponds to the number of ECG sensing electrodes that aredesired to be monitored. A suitable type of A/D converter that can becascaded or daisy chained in the manner described above is a MAX 11040K(24 bit) or Max11060 (16 bit) ADC available from Maxim IntegratedProducts of Sunnyvale Calif., although other analog-to digitalconverters available from other companies may alternatively be used.

In accordance with this embodiment, the at least one processor 410 maysend a control signal to each of the plurality of A/D converters 1110a-p to sample a respective signal at substantially the same period oftime, and send the sampled and converted digital signal to the next A/Dconverter in the chain, at a subsequent time. Ultimately, the last A/Dconverter 1110 p provides the serial bitstream to the at least oneprocessor 410. Dependent upon which of the plurality of ECG sensingelectrodes are selected to be paired with one another, the at least oneprocessor 410 extracts the digital signals corresponding to the twoselected digital signals (typically corresponding to the same timeperiod), inverts one of them, and digitally sums the signals,effectively performing the same functionality as the differentialinstrumentation amplifiers described with respect to FIGS. 2A-C and 3above. The selection, inversion, and summing of selected pairs ofdigital signals may be performed for any pairing of ECG sensingelectrodes, and corresponding to the same, or different time periods.The digitally summed signals may then be processed to monitor thepatient's ECG signals and/or to detect any arrhythmic cardiac condition.

It should be appreciated that where the signal acquisition circuitsdescribed above with respect to FIGS. 9-11 are used with an electrodesystem associated with a wearable ambulatory medical device, each ofthese signal acquisition circuits not only permit the monitoring andanalysis of ECG signals from any pairing of ECG sensing electrodes thatare associated with the wearable ambulatory medical device, but theyalso permit the signal of any of the plurality of ECG sensing electrodes10 a-p to be paired with the signal from another source, such as awireless ECG sensing electrode. For example, a wireless ECG sensingelectrode may be provided that includes an A/D converter, such as any ofA/D converters 1010 a-p or 1110 a-p described above, that is coupled toa wireless transmitter or transceiver and can communicate with the atleast one processor 410 via a wireless communication protocol such asBluetooth, ZigBee, Wireless USB, Wireless Ethernet, GSM, etc. The signalfrom the wireless ECG sensing electrode may then be paired with any ofthe signals from each of ECG sensing electrodes 10-p that are associatedwith the wearable ambulatory medical device, where it is desirable to doso. In this manner, if additional cardiac information is desired,additional wireless ECG sensing electrodes may be placed on thepatient's body, and those signals monitored and analyzed. Indeed, insome embodiments, each of the ECG sensing electrodes need not beassociated with a garment that is worn by the patient, but each of theECG sensing electrodes may be self adhesive wireless ECG sensingelectrodes that are simply placed, as desired, on the patient's body.

Having thus described several aspects of at least one embodiment of thisinvention, it is to be appreciated various alterations, modifications,and improvements will readily occur to those skilled in the art. Suchalterations, modifications, and improvements are intended to be part ofthis disclosure, and are intended to be within the scope of theinvention. Accordingly, the foregoing description and drawings are byway of example only.

What is claimed is:
 1. An ambulatory medical device, comprising: aplurality of electrodes configured to be disposed at spaced apartpositions about a body of a patient; an electrode signal acquisitioncircuit having a plurality of inputs, each respective input of theplurality of inputs being electrically coupled to a respective electrodeof the plurality of electrodes, the electrode signal acquisition circuitbeing configured to sense a respective signal provided by a plurality ofdifferent pairings of the plurality of electrodes; and a monitoringcircuit, electrically coupled to an output of the electrode signalacquisition circuit, the monitoring circuit being configured to analyzethe respective signal provided by each of the plurality of differentpairings and to instruct the electrode signal acquisition circuit toselect at least one of the plurality of different pairings to monitorbased upon a quality of the respective signal provided by the selectedat least one of the plurality of different pairings and a phasedifference between the respective signal provided by the selected atleast one of the plurality of different pairings and the respectivesignal provided by another selected at least one of the plurality ofdifferent pairings.
 2. The ambulatory medical device of claim 1, furthercomprising a garment that is configured to be worn about the body of thepatient, wherein the plurality of electrodes are integrated into thegarment.
 3. The ambulatory medical device of claim 2, wherein theplurality of electrodes are ECG sensing electrodes, and wherein themonitoring circuit is a cardiac monitoring and arrhythmia detectioncircuit.
 4. The ambulatory medical device of claim 3, wherein theplurality of ECG sensing electrodes includes at least three ECG sensingelectrodes.
 5. The ambulatory medical device of claim 4, wherein theplurality of ECG sensing electrodes are not all located in a commonplane.
 6. The ambulatory medical device of claim 3, further comprising:a plurality of therapy electrodes integrated into the garment andconfigured to deliver a defibrillating shock to the body of the patientin response to detection of a treatable cardiac arrhythmia by thecardiac monitoring and arrhythmia detection circuit.
 7. The ambulatorymedical device of claim 1, wherein the monitoring circuit is configuredto analyze the respective signal provided by each of the plurality ofdifferent pairings and to instruct the electrode signal acquisitioncircuit to select at least three of the plurality of different pairingsto monitor based upon the quality of the respective signal provided byeach of the selected at least three of the plurality of differentpairings, the phase difference between the respective signal provided byeach of the selected at least three of the plurality of differentpairings, and the position of the respective electrodes of the selectedat least three of the plurality of different pairings relative to thebody of the patient.
 8. The ambulatory medical device of claim 1,further comprising a garment that is configured to be worn about thebody of the patient, wherein the plurality of electrodes are integratedinto the garment, and wherein the plurality of electrodes includes atleast four ECG sensing electrode that are not all located in a commonplane.
 9. The ambulatory medical device of claim 1, wherein themonitoring circuit is configured to analyze the respective signalprovided by each of the plurality of different pairings and to instructthe electrode signal acquisition circuit to select at least three of theplurality of different pairings to monitor based upon the quality of therespective signal provided by the selected at least one of the pluralityof different pairings, the phase difference between the respectivesignal provided by each of the selected at least three of the pluralityof different pairings, and the plane defined by the respectiveelectrodes of the selected at least three of the plurality of differentpairings.
 10. The ambulatory medical device of claim 1, wherein themonitoring circuit is configured to analyze the respective signalprovided by each of the plurality of different pairings and to instructthe electrode signal acquisition circuit to select at least three of theplurality of different pairings to monitor based upon the quality of therespective signal provided by the selected at least one of the pluralityof different pairings, the phase difference between the respectivesignal provided by each of the selected at least three of the pluralityof different pairings, and the position of the respective electrodes ofthe selected at least three of the plurality of different pairingsrelative to the body of the patient and the cardiac cycle of the heartof the patient.
 11. The ambulatory medical device of claim 1, whereinthe monitoring circuit is configured to analyze the respective signalprovided by each of the plurality of different pairings and to instructthe electrode signal acquisition circuit to select at least three of theplurality of different pairings to monitor based upon the quality of therespective signal provided by each of the selected at least three of theplurality of different pairings, the phase difference between therespective signal provided by each of the selected at least three of theplurality of different pairings, and at least one of the position of therespective electrodes of the selected at three of the plurality ofdifferent pairings relative to the body of the patient, the planedefined by the respective electrodes of the selected at least three ofthe plurality of different pairings, and the cardiac cycle of the heartof the patient.
 12. The ambulatory medical device of claim 11, whereinthe monitoring circuit is configured to instruct the electrode signalacquisition circuit to select at least two of the selected at leastthree of the plurality of different pairings to monitor during a firsttime interval and to select a different at least two of the selected atleast three of the plurality of different pairings to monitor during asecond time interval.
 13. The ambulatory medical device of claim 1,wherein the plurality of electrodes are ECG sensing electrodes, andwherein the monitoring circuit is a cardiac monitoring and arrhythmiadetection circuit.
 14. The ambulatory medical device of claim 13,wherein the plurality of ECG sensing electrodes includes at least threeECG sensing electrodes.
 15. The ambulatory medical device of claim 1,wherein the electrode signal acquisition circuit includes: a pluralityof differential amplifiers each having a pair of inputs and an output,the plurality of differential amplifiers including a respectivedifferential amplifier for each unique pairing of the plurality ofelectrodes; and a selection circuit configured to select at least oneoutput of the plurality of differential amplifiers to provide to themonitoring circuit.
 16. An ambulatory medical device, comprising: aplurality of electrodes configured to be disposed at spaced apartpositions about a body of a patient; an electrode signal acquisitioncircuit having a plurality of inputs, each respective input of theplurality of inputs being electrically coupled to a respective electrodeof the plurality of electrodes, the electrode signal acquisition circuitbeing configured to sense a respective signal provided by a plurality ofdifferent pairings of the plurality of electrodes; and a monitoringcircuit, electrically coupled to an output of the electrode signalacquisition circuit, the monitoring circuit being configured to analyzethe respective signal provided by each of the plurality of differentpairings and to instruct the electrode signal acquisition circuit toselect at least one of the plurality of different pairings to monitorbased upon a quality of the respective signal provided by the selectedat least one of the plurality of different pairings and a plane definedby the respective electrodes of the selected at least one of theplurality of different pairings.
 17. An ambulatory medical device,comprising: a plurality of electrodes configured to be disposed atspaced apart positions about a body of a patient; an electrode signalacquisition circuit having a plurality of inputs, each respective inputof the plurality of inputs being electrically coupled to a respectiveelectrode of the plurality of electrodes, the electrode signalacquisition circuit being configured to sense a respective signalprovided by a plurality of different pairings of the plurality ofelectrodes; and a monitoring circuit, electrically coupled to an outputof the electrode signal acquisition circuit, the monitoring circuitbeing configured to analyze the respective signal provided by each ofthe plurality of different pairings and to instruct the electrode signalacquisition circuit to select at least one of the plurality of differentpairings to monitor based upon a position of the respective electrodesof the selected at least one of the plurality of different pairingsrelative to the body of the patient and a cardiac cycle of the heart ofthe patient.
 18. An ambulatory medical device, comprising: a pluralityof electrodes configured to be disposed at spaced apart positions abouta body of a patient; an electrode signal acquisition circuit having aplurality of inputs, each respective input of the plurality of inputsbeing electrically coupled to a respective electrode of the plurality ofelectrodes, the electrode signal acquisition circuit being configured tosense a respective signal provided by a plurality of different pairingsof the plurality of electrodes; and a monitoring circuit, electricallycoupled to an output of the electrode signal acquisition circuit, themonitoring circuit being configured to analyze the respective signalprovided by each of the plurality of different pairings and to instructthe electrode signal acquisition circuit to select at least one of theplurality of different pairings to monitor based upon at least one of aquality of the respective signal provided by the selected at least oneof the plurality of different pairings, a phase difference between therespective signal provided by the selected at least one of the pluralityof different pairings and the respective signal provided by anotherselected at least one of the plurality of different pairings, a positionof the respective electrodes of the selected at least one of theplurality of different pairings relative to the body of the patient, aplane defined by the respective electrodes of the selected at least oneof the plurality of different pairings, and a cardiac cycle of a heartof the patient; wherein the electrode signal acquisition circuitincludes a selection circuit having a plurality of inputs and aplurality of outputs, each respective input of the plurality of inputsof the selection circuit being electrically coupled to a respective oneof the plurality of electrodes, and a plurality of differentialamplifiers, each respective differential amplifier having a pair ofinputs and an output, each respective input of the pair of inputs beingelectrically coupled to a respective one of the plurality of outputs ofthe selection circuit, each respective output of the plurality ofdifferential amplifiers providing an output signal corresponding to adifference between the pair of inputs of the respective differentialamplifier.
 19. An ambulatory medical device, comprising: a plurality ofelectrodes configured to be disposed at spaced apart positions about abody of a patient; an electrode signal acquisition circuit having aplurality of inputs, each respective input of the plurality of inputsbeing electrically coupled to a respective electrode of the plurality ofelectrodes, the electrode signal acquisition circuit being configured tosense a respective signal provided by a plurality of different pairingsof the plurality of electrodes; and a monitoring circuit, electricallycoupled to an output of the electrode signal acquisition circuit, themonitoring circuit being configured to analyze the respective signalprovided by each of the plurality of different pairings and to instructthe electrode signal acquisition circuit to select at least one of theplurality of different pairings to monitor based upon at least one of aquality of the respective signal provided by the selected at least oneof the plurality of different pairings, a phase difference between therespective signal provided by the selected at least one of the pluralityof different pairings and the respective signal provided by anotherselected at least one of the plurality of different pairings, a positionof the respective electrodes of the selected at least one of theplurality of different pairings relative to the body of the patient, aplane defined by the respective electrodes of the selected at least oneof the plurality of different pairings, and a cardiac cycle of a heartof the patient; wherein the electrode signal acquisition circuitincludes an analog multiplexor having a plurality of inputs and anoutput, each of the plurality of inputs being electrically coupled to arespective one of the plurality of electrodes, and an analog-to-digitalconverter having an input electrically coupled to the output of theanalog multiplexer.
 20. The ambulatory medical device of claim 19,wherein the analog-to digital converter has a sampling rate that is atleast N times a desired sampling rate of a signal provided by each ofthe plurality of electrodes, where N is the number of the plurality ofelectrodes that are to be monitored.
 21. The ambulatory medical deviceof claim 19, wherein the monitoring circuit includes at least oneprocessor configured to receive a first digital signal corresponding toa first electrode of the plurality of electrodes and a second digitalsignal corresponding to a second electrode of the plurality ofelectrodes, to invert one of the first and second digital signals andsum the inverted one of the first and second digital signals with theother of the first and second digital signals to analyze the respectivesignal provided by a pairing of the first electrode and the secondelectrode.
 22. An ambulatory medical device, comprising: a plurality ofelectrodes configured to be disposed at spaced apart positions about abody of a patient; an electrode signal acquisition circuit having aplurality of inputs, each respective input of the plurality of inputsbeing electrically coupled to a respective electrode of the plurality ofelectrodes, the electrode signal acquisition circuit being configured tosense a respective signal provided by a plurality of different pairingsof the plurality of electrodes; and a monitoring circuit, electricallycoupled to an output of the electrode signal acquisition circuit, themonitoring circuit being configured to analyze the respective signalprovided by each of the plurality of different pairings and to instructthe electrode signal acquisition circuit to select at least one of theplurality of different pairings to monitor based upon at least one of aquality of the respective signal provided by the selected at least oneof the plurality of different pairings, a phase difference between therespective signal provided by the selected at least one of the pluralityof different pairings and the respective signal provided by anotherselected at least one of the plurality of different pairings, a positionof the respective electrodes of the selected at least one of theplurality of different pairings relative to the body of the patient, aplane defined by the respective electrodes of the selected at least oneof the plurality of different pairings, and a cardiac cycle of a heartof the patient; wherein the electrode signal acquisition circuitincludes a plurality of analog-to-digital converters, each respectiveanalog-to-digital converter having a respective input that iselectrically coupled to a respective one of the plurality of electrodes.23. The ambulatory medical device of claim 22, wherein each of theplurality of analog-to-digital converters is connected to another of theplurality of analog-to-digital converters by a serial bus.
 24. A methodof monitoring ECG signals, the method comprising acts of: selecting,from among a plurality of ECG sensors, a plurality of different pairingsof ECG sensors; analyzing a respective ECG signal provided by each ofthe plurality of different pairings; identifying at least one of theplurality of different pairings to monitor based upon at least one of aquality of the respective ECG signal provided by the identified at leastone of the plurality of different pairings, a phase difference betweenthe respective ECG signal provided by the identified at least one of theplurality of different pairings and the respective ECG signal providedby another identified at least one of the plurality of differentpairings, a position of respective ECG sensors of the identified atleast one of the plurality of different pairings relative to a body of apatient, a plane defined by the respective ECG sensors of the identifiedat least one of the plurality of different pairings, and a cardiac cycleof a heart of the patient; and monitoring the identified at least one ofthe plurality of different pairings; wherein the act of identifying atleast one of the plurality of different pairings to monitor is basedupon the quality of the respective ECG signal provided by the identifiedat least one of the plurality of different pairings and the phasedifference between the respective ECG signal provided by the identifiedat least one of the plurality of different pairings and the respectiveECG signal provided by the other identified at least one of theplurality of different pairings.
 25. The method of claim 24, wherein theact of selecting the plurality of different pairings of ECG sensors fromamong the plurality of ECG sensors includes an act of selecting, fromamong the plurality of ECG sensors, each unique pairing of ECG sensors;and wherein the act of analyzing the respective ECG signal provided byeach of the plurality of different pairings includes analyzing therespective ECG signal provided by each unique pairing of ECG sensors.26. The method of claim 25, wherein the act of monitoring includes:monitoring the identified at least one of the plurality of differentpairings to detect a cardiac arrhythmia.
 27. The method of claim 26,further comprising acts of: detecting the cardiac arrhythmia responsiveto the act of monitoring; determining that the detected cardiacarrhythmia is a type of cardiac arrhythmia that can be treated byapplying defibrillation to the body of the patient; and applying atleast one defibrillation pulse to the body of the patient.
 28. Themethod of claim 26, further comprising acts of: detecting the cardiacarrhythmia responsive to the act of monitoring; selecting at least oneadditional pairing of ECG sensors in response to detecting the cardiacarrhythmia and analyzing the respective ECG signal provided by the atleast one additional pairing; determining that the detected cardiacarrhythmia is also present on the respective ECG signal of the at leastone additional pairing; determining that the detected cardiac arrhythmiais a type of cardiac arrhythmia that can be treated by applyingdefibrillation to the body of the patient; and applying at least onedefibrillation pulse to the body of the patient.
 29. The method of claim26, further comprising acts of: detecting the cardiac arrhythmiaresponsive to the act of monitoring; selecting at least one additionalpairing of ECG sensors in response to detecting the cardiac arrhythmiaand analyzing the respective ECG signal provided by the at least oneadditional pairing; determining that the detected cardiac arrhythmia isalso present on the respective ECG signal of the at least one additionalpairing; and increasing a confidence level that the cardiac arrhythmiahas been detected.
 30. The method of claim 26, further comprising actsof: detecting the cardiac arrhythmia responsive to the act ofmonitoring; selecting at least one additional pairing of ECG sensors inresponse to detecting the cardiac arrhythmia and analyzing therespective ECG signal provided by the at least one additional pairing;determining that the detected cardiac arrhythmia is not present on therespective ECG signal of the at least one additional pairing; anddecreasing a confidence level that the cardiac arrhythmia has beendetected.
 31. The method of claim 24, wherein the acts of selecting,analyzing, and identifying are repeated at periodic intervals.
 32. Themethod of claim 24, wherein the plurality of ECG sensors are integratedin a garment that is worn about the body of the patient, and wherein theacts of selecting, analyzing, and identifying are performed each timethe garment is placed about the body of the patient.
 33. The method ofclaim 24, wherein the plurality of ECG sensors are integrated in agarment that is worn about the body of the patient, the method furthercomprising an act of: detecting strenuous physical activity of thepatient; and repeating the acts of selecting, analyzing, and identifyingin response to the act of detecting the strenuous activity of thepatient.
 34. The method of claim 24, further comprising acts of:determining that the quality of the respective ECG signal provided by afirst pairing of ECG sensors of the identified at least one of theplurality of different pairings is below a determined threshold;selecting another paring of ECG sensors to replace the first pairing ofECG sensors; and monitoring the other pairing of ECG sensors.
 35. Themethod of claim 24, further comprising acts of: determining, from thequality of the respective ECG signal provided by a first pairing of ECGsensors of the identified at least one of the plurality of differentpairings, that one or more of the ECG sensors of the first pairing mayhave at least partially lost contact with the body of the patient;selecting another paring of ECG sensors to replace the first pairing ofECG sensors; and monitoring the other pairing of ECG sensors.
 36. Amethod of monitoring ECG signals, the method comprising acts of:selecting, from among a plurality of ECG sensors, a plurality ofdifferent pairings of ECG sensors; analyzing a respective ECG signalprovided by each of the plurality of different pairings; identifying atleast one of the plurality of different pairings to monitor based uponat least one of a quality of the respective ECG signal provided by theidentified at least one of the plurality of different pairings, and aphase difference between the respective ECG signal provided by theidentified at least one of the plurality of different pairings and therespective ECG signal provided by another identified at least one of theplurality of different pairings, a position of respective ECG sensors ofthe identified at least one of the plurality of different pairingsrelative to a body of a patient, a plane defined by the respective ECGsensors of the identified at least one of the plurality of differentpairings, and a cardiac cycle of a heart of the patient; and monitoringthe identified at least one of the plurality of different pairings;wherein the act of identifying at least one of the plurality ofdifferent pairings to monitor is based upon the quality of therespective ECG signal provided by the identified at least one of theplurality of different pairings and the plane defined by the respectiveECG sensors of the identified at least one of the plurality of differentpairings.
 37. A method of monitoring ECG signals, the method comprisingacts of: selecting, from among a plurality of ECG sensors, a pluralityof different pairings of ECG sensors; analyzing a respective ECG signalprovided by each of the plurality of different pairings; identifying atleast one of the plurality of different pairings to monitor based uponat least one of a quality of the respective ECG signal provided by theidentified at least one of the plurality of different pairings, and aphase difference between the respective ECG signal provided by theidentified at least one of the plurality of different pairings and therespective ECG signal provided by another identified at least one of theplurality of different pairings, a position of respective ECG sensors ofthe identified at least one of the plurality of different pairingsrelative to a body of a patient, a plane defined by the respective ECGsensors of the identified at least one of the plurality of differentpairings, and a cardiac cycle of a heart of the patient; and monitoringthe identified at least one of the plurality of different pairings;wherein the act of identifying at least one of the plurality ofdifferent pairings to monitor is based upon the position of respectiveECG sensors of the identified at least one of the plurality of differentpairings relative to the body of the patient and the cardiac cycle ofthe heart of the patient.